I've been wondering about how much of the capacity of the 46 gallon Transfer Flow fuel tank is usable so I searched for discussions on this and ran across this thread.
This particular post caught my attention:
Originally Posted by flyfisher
I just don't understand why anyone wants to see how far they can get on a tank of diesel or gas before the damn light comes on. And I don't buy the argument that the less fuel in the tank means less weight to push resulting in better mileage...We didn't buy these rigs because we were concerned about mileage.....
fill it up when it's half empty and worry about other issues...like what to do when you've gone through all the Bourbon and everyone else in the campground is drinking white wine.
The general sentiment about how much of an emergency it is when you're out of bourbon when there is only white wine available in camp is no doubt universally shared among campers everywhere. But sadly it seems that this post about why fuel calculations are important is indicative of a general fuzziness about some of the concepts regarding fuel planning among our Sportsmobiling friends (and other drivers).
Here are some thoughts on the matter and hopefully they will be useful. Please accept my apologies in advance for the long-windedness of the post. It's a big subject.
The point is not how far you can go before you run out of fuel or before the low fuel light comes on, or whatever variations.
The reason for knowing how much usable fuel you carry is so you can take advantage of the actual size of your tank.
For instance, if a stock fuel tank holds 37 gallons and the Transfer Flow holds 46, you theoretically have 9 extra gallons in the TF tank vs the stock tank. In this theoretical world, if you were previously carrying two 5-gallon jerry cans full of fuel when you had a 37-gallon tank, you no longer have to carry those jerry cans because that fuel is already in your tank. (see the section on jerry cans at the bottom of this post)
But this theoretical world only takes effect if you have some degree of confidence, not just in how many gallons your tank holds (capacity), but in how much fuel of that fuel can really be sucked into the engine and burned to make power. This concept is known as "usable fuel". Usable fuel is essential to proper fuel management. It's important enough that airplanes are tested for this as part of the certification process and both capacity and usable fuel are numbers that are part of the published specification for the airplane. Unusable fuel, in an airplane, becomes part of planning for weight and balance. Usable fuel is used to determine range.
Fuel management in aviation a useful illustration
When you learn to fly a plane, you're taught about managing the engine's power for both speed and fuel economy. Typically, you're at least taught about achieving "best power" and also achieving "best economy." The airplane has published performance numbers (speed and fuel consumption, as well as how to set the fuel flows and other power-related controls) for both of those. In addition, you learn about fuel reserves and planning around them. Upon landing, the aviation regulations say you must have at least 30 minutes of fuel remaining for day VFR (visual flight rules) operation and at least 45 minutes of fuel remaining for night and/or IFR (instrument flight rules) operation. When you're planning your fuel reserves, those reserves must be calculated to include having travelled to your planned alternate airport. (Planned alternate airports are required for some IFR flights, depending on forecast conditions at your originally intended destination.)
Flight planning, for fuel purposes, includes the factors above, plus condition-of-flight issues like headwinds and tailwinds (these are derived from forecasts and pilot reports), expected wait times before takeoff (assessed using fuel-consumption information available in the plane's operating handbook and knowledge of the size and traffic conditions at your departure airport), time-and-fuel-to-climb to your enroute altitude (available as a graph in your plane's operating handbook which factors in elevation of your departing airport, local temperatures, etc), performance at altitude, etc.
Once all of these factors are assessed and calculated for, you can compare the expected total fuel burn for your planned trip leg, plus the required reserves at your planned power setting, to the usable fuel amount published in the plane's operating handbook. (All of this planning may seem like a staggering hassle, but it becomes second-nature and is pretty easy.)
As a final thought on fuel management as applied to operating airplanes (and shortly below I'll apply it to SMBs), a prudent pilot may wish to plan for MORE than the legally required fuel reserves in the event that things don't work out exactly as planned (this never happens in the real world, I know, but one can always pretend...). And finally (2), when you're operating an airplane you have considerations about "useful load" which involves things like how much fuel you can carry vs "payload" which is passenger and cargo weight. Further illustrations about this subject are waiting for you as you progress in your flying lessons should you be ambitious enough to take some.
Normal fuel management concepts for road vehicles
Fortunately, aviation's "useful load" concept can largely be dispensed with for the purposes of driving. You normally can carry fuel without consideration for weight, unless you have some really massive tank that will make a difference to fuel mileage if hauling around a full tank. This will not apply to most of us. And if it does, you already know or should know who you are. Worrying about the weight of the fuel in road vehicles is only for hypermilers and if you're driving a Sportsmobile or similar, you are not a hypermiler.
For fuel planning when in civilized parts (by "civilized parts" I mean fuel is generally available along your route), these are the things I take into account:
1) Fuel availability
- I don't want to get surprised by a future potential fuel stop not having fuel
2) Routing flexibility
- I want to have enough fuel that if I feel like taking an unplanned side trip I'll generally be able to do it. There is also the possibility that a road closure or other mishap will force a re-route or a higher expected fuel burn and I want to have enough fuel on board to be flexible about it.
3) Fuel price
Is fuel a lot more expensive at this station or town? A lot cheaper? Fuel price may be a major factor, like when you know that 20 miles up the road the fuel is 50-cents-a-gallon cheaper and you have 200 miles of fuel on board, or it may be a very minor factor, like when you figure that you're likely to run out of fuel or you won't be able to do your planned excursion if you don't buy this overpriced fuel right now, etc. Note that $0.10 per gallon may make a difference of $3.00 if you need 30 gallons of fuel. Even if it's a dollar a gallon too pricey but it's the only game in town and you have reason to believe you must have more fuel to achieve your goals, it's not hard to imagine that paying $40 or $50 extra for some fuel is a better choice than trying to push the van to the next available fuel station. Or abandoning the van while you go in search of fuel. If for some reason you're really being gouged for fuel, maybe it is prudent to pay it (as in you're escaping from a natural disaster and fuel is more valuable than money) or it may be more prudent to buy what you need from Bubba to get to the next corporate fuel stop with more-usual prices (notice I didn't say "reasonable prices"). I've never had to worry about either the zombie apocalypse or Bubba's Last Gas(p) and you probably haven't either but you never know when you might get caught up in nightmare from a Roland Emmerich film, so take these things under advisement as you see fit.
4) Fuel quality
Do I have reason to believe that the fuel at this particular station is contaminated or otherwise substandard in any way? I recently ran across a station that offered 85 octane, but my vans are supposed to use a minimum of 87 octane. Fortunately, this station had 87 and 91 also, but what if there was a station that only had 85? At that point, the other considerations above come into play regarding whether to purchase fuel. And of course if you believe you're likely to take on a load of bad fuel, you might consider how to plan around this even if you think you have no other choice. Again, this has never happened to me or probably you either, so I won't get into ways to deal with it, but it is a consideration nonetheless.
- I generally want to be able to fill my tank with only a single swipe of my credit card. (This typically means I want to require less than $100 worth of fuel to top off the tank for any given fillup. At $4-ish a gallon, that's 20-ish gallons.) Another consideration is that if I have to stop at all, I might as well put in fuel. Especially since the logistical considerations (i.e. how long it takes to fill the tank) tend to be minimal when you don't need to take on a lot of fuel. And if I already have plenty of fuel, convenience may mean I don't need to stop for fuel -- though all of the first 4 considerations above need to be taken into account too.
With all of these considerations in mind and all other things being equal (which of course they never are): I typically fill up by the time I get to a half tank. But what is a half tank? Is it when the gauge shows 1/2 tank remaining? Or is it when you have really used 1/2 of your usable fuel? For our general purposes in on-road civilized-parts planning, using the gauge is fine. But if you know that you have only actually used 1/3 of a tank when your gauge says you've used 1/2 tank, you "might" take that into consideration. But since you're a conservative fuel planner, you're going to fill up at 1/2 tank indicated on the gauge if it's convenient to do so. (More on this in a moment.)
Let's say you're driving on a highway and a sign says "next fuel 120 miles." There are a couple of places I know of like that. So do you look at your fuel gauge and figure "well I get 10mpg and I have at least 15 gallons on board, so I should be fine" and then stomp on the accelerator to blast on down the road to the 120-miles-away next fuel stop? Maybe, but only if you don't want to get bit by considerations 1 thru 4, listed above.
If you're driving out west where the distances can be large, you want to keep your tank close to full. This is "at all times", and especially when the weather is extreme, which is "a lot of the time."
Serious fuel planning for adventure travel
Let's apply aviation concepts to fuel planning for adventure-van trips where availability of fuel is a concern. As indicated above, we don't have to worry about useful load and fuel weight. We're just going to figure out how to get the most utility out of our existing fuel tank and the least white-knuckle fear about whether you're likely to run out of fuel on a given excursion.
Remember: airplanes may purposely carry less-than-full fuel because most planes can't carry full fuel and full passengers/payload You adjust one or the other to fit the mission. If you have to carry a heavy load of passengers or stuff and you can't fill the tanks because of weight and/or balance, you take on less fuel and plan shorter legs. If you must go a long way without a fuel stop, then you must adjust your payload to fit the amount of fuel you must carry.
For normal road trips in your van, this isn't a consideration because you can top off frequently.
But once you're going to do a long off-road trip, the exact amount of fuel you need may become an issue.
Planning requires knowing the distances you need to travel between fuel stops, knowing your expected fuel consumption, and making decisions about reserves and other strategies that help you incorporate a margin for error.
So let's plan a hypothetical trip where you can buy gas, then have to drive 70 miles on paved roads to the trailhead before beginning the off-road portion of the trip. Once off-road, you'll be driving 25 miles on gravel, probably in 4WD-high. Then you'll get into a mix of 4-low and 4-high for some serious exploration. Let's say you've got 60 miles through a high mountain pass, then another 50 miles of high-country trails, then a 40-mile cross country descent into another back country area where at least there are paved roads. From there, you'll be able to get to a dependable gas station in 35 miles.
So this trip is 225 miles between fuel stops. No problem. At 10mpg, it will only take 22.5 gallons and your 37 gallon tank will have 14.5 gallons left.
But we haven't really fully accounted for the fuel burn yet. For instance, we haven't factored idling into the equation. Are you going to be idling for any significant length of time either en route or in camp? My ScanGauge says that the V-10 in my van uses just under 1.2 gallons per hour while idling. Maybe it's best to round up to 1.5 gallons per hour. So if you think you might be idling for an hour, total, along your way each day, plus a total of an hour each day in camp (I'm not sure why you would, but let's say you need to do so to support systems on your rig). So if it's a 2-day trip we need to plan for 6 gallons for idling. That means our trip requires 22.5 + 6 gallons, or 28.5 gallons total. Leaving a hypothetical 8.5 gallons of fuel on board our 37-gallon-capacity van.
Are these calculations right? Probably not. To do this propertly we need to calculate fuel use a little more precisely. Like so:
We must assign fuel consumption to each segment of the trip based on our expectations and knowledge. Maybe we can count on 14mpg on smooth level paved roads. But maybe only the first 10 miles meet that criteria. From there, we begin a series of climbs and descents that combine for 8000 feet of climbs (on slow switchbacks) and an equal amount of descents. But the climbs are more miles (just to make things tougher). So we should break it out with a greater degree of precision. Here is a spreadsheet for doing this kind of calculation, where each segment is broken out by condition, distance and estimated fuel burn for that segment:
(I also tried to attached the actual spreadsheet but no extensions that I tried were allowed. Apologies. It's pretty easy to build this yourself if you need it.)
So calculated fuel consumption for this trip = 23 gallons not including idling fuel.
Hey, not bad, just a bit more than than our previously-eyeballed 22.5 gallons! But that was just luck, the way it turned out. Don't depend on your guesswork when fuel calculations are critical.
Plus our expected idling usage of 6 gallons, we're now at 29 gallons of projected fuel usage.
Let's factor in at least 10% as a margin for error and we're at 32 gallons projected based on calculated needs plus a fudge factor, but not including reserves and yes I rounded up when I added the 10%. Because that's how you do it, you round up your expectations of fuel use. And if you're almost not quite going to make it, you try to do your calculations more precisely and you also carry extra fuel as needed to make up for your expectations of fuel needs.
Please remember that as always with calculations, GIGO: Garbage In, Garbage Out. Unless we've previously painstakingly kept records of our fuel use in various situations, like 4WD rock-crawling etc we may have no reasonable basis for using these numbers in our calculations. Unless we've previously driven these routes and KNOW how our rig burns fuel on them, we may have no reasonable expectations that our projected fuel-burn estimates are right. So we must leave a large margin for error if we don't wish to risk running out of fuel.
We can know a lot about the conditions of the road by using appropriate maps. We can usually make rough determinations about elevation changes and terrain based on map info. This information helps you apply your previously-gathered information about fuel consumption in various circumstances. We can know about road conditions because of driver reports, news reports, weather reports, etc.
In any case, based on the assumptions in our hypothetical trip planning above, we probably don't have enough fuel margin for comfort (using those numbers and a 37 gallon tank with a (hypothetical) 36.2 usable gallons on board).
So we may want to sharpen our pencils, look at the fuel burn more critically, consider how we might conserve fuel better, whether we need to carry extra fuel, etc. A lot of how we solve this problem depends on our knowledge of our vehicles fuel consumption under varying conditions and with varying degrees of lead-footedness.
Final decisions after doing your fuel planning
Okay, we've decided per the last section that we have a projected fuel burn of 32 gallons. (Notice how I keep rounding consumption figures up? That's because I don't want to run out of gas.)
That means, based on our 37 gallon tank that we will have less than 5 gallons of fuel left in the tank when we buy gas at the end of that run.
But wait... do we really know that those 4+ gallons are actually there and can be burned by our truck?
Unfortunately, without substantially paying attention to the "usable fuel" concept, the answer is we probably don't know if that fuel is there or if it is available.
Fuel tank volumes are often wrong. Fuel availability towards the bottom of the tank is affected by the location of the pickup tube, the attitude of the vehicle and maybe other factors.
THINGS YOU CAN DO TO AID FUEL PLANNING
Record how much gas you need for any given fuel gauge position
At some point when each of these gauge events happen, get a fill up and write down the results. Periodically confirm the results when you buy fuel.
Fuel gauge event > Gallons needed
1) As soon as the gauge starts moving down after a fillup tank takes on ___ gallons
2) 3/4 tank tank takes on ___ gallons
3) 1/2 tank tank takes on ___ gallons
4) 1/4 tank tank takes on ___ gallons
5) gauge on E tank takes on ___ gallons
6) fuel warning light comes on tank takes on ___ gallons
You might add another column that tells you gallons remaining when at the given gauge event. You'll be able to calculate that when you know your usable fuel. Remember, usable fuel equals the most you've ever put in your tank in a single fill. Unless you've run it dry (which may not be a good thing to do) presuming you have any more than the proven amount you've put in your tank before may be wrong.
Correlate your fuel gauge events with your fuel planning
Knowing that you're at 1/4 tank and that you have at least 6 gallons until it is on empty, and you have more after that, is useful information when you're planning your fuel stops. If nothing else, it may corroborate your other planning efforts. For instance:
If your calculations show that you should be indicating a 1/2 tank by point X in your extended journey between fuel stations, and it turns out that you are in fact indicating 1/2 tank at point X, you have some reassurance that your planning was correct. If you get to point X with more or less fuel indicated, you may have some soul-searching to do. Especially if you're indicating significantly less fuel than you expeced to be indicating. Maybe a re-route for fuel is in order, if that's possible. Or maybe it's time to whiten those knuckes and hope for the best. But if you've done your planning well, with lots of reserves, your knuckles can stay flesh-colored and you can continue your journey, feeling relaxed and worry-free.
The fuel-usage game
Once you have recorded these fuel-gauge events and fuel-needed, make a game out of it by predicting at every fuel stop exactly how much fuel you will need in order to top off the tank.
There are two ways to do this.
One is based on the fuel gauge event. Based on your previous records and the current gauge position, how much fuel should you need to top off the tank?
The other is based on the distance travelled and your expectations of fuel economy for your given loading and driving conditions.
Use both methods all the time. That way you are always dealing with fresh data about your rig's current fuel consumption. Plus you're finding out whether your assumed trip averages are correct and whether your ScanGauge's calculations are correct, etc.
In any case, if your expectations of fuel-needs are wrong at any point, you have an opportunity to figure out why and either adjust your future expectations or fix whatever went wrong.
HOW MUCH USABLE FUEL DO YOU REALLY HAVE?
There is only one way to know for sure how much usable fuel you have on board your rig: You only know how much usable fuel you have based on actual demonstration. I.e. if you've never put more than 30 gallons of fuel in your tank, then you don't know if you have any more usable fuel than 30 gallons, no matter what the tank's theoretical capacity is.
how to test for usable fuel
- The ultimate useable fuel test is this:
Carry an extra gallon or two of fuel in a separate can, run the tank dry, pour in the extra fuel. Write down your odometer setting so you can determine exactly how far is is to the nearest fuel station where you will now fill your tank. Get to the station, write down your odometer setting and fill the tank. While filling the tank, calculate how many miles you travelled from where you ran out of fuel to the station.
Add the number of gallons it took to flll the tank to the extra fuel you poured in earlier (when you first ran out of fuel) for total gallons, then subtract the amount of fuel you expected to have consumed in the trip from where you ran out of gas to where you filled up. E.g:
Extra fuel put in tank 2 gallons
Fill-up gallons 43.8
Total fuel in: 45.8 gallons
Distance from out-of-fuel to station: 6 miles
Usual gas miles under the driving conditions from out-of-fuel to the station: 11mpg. 11/6 = 1.83 gallons.
So your tank holds 44 useable gallons.
At least it holds 44 useable gallons of fuel this time, based on the levelness of the vehicle when filling, the temperature of the fuel (often affected by season as well as a morning vs late afternoon fillup), how temperamental your rig is about letting the tank get full, the condition and/or calibration of the fill nozzle, etc).
- (By the way, if you must use rounding, never round fuel capacity up, always round it down -- i.e 45.8 gallons does not round to 46 gallons. This isn't elementary school and the usual rules of rounding don't apply. In the real world, you would truncate the number and say that you added 45 gallons. This will distort your perception of how much fuel you carry in a good direction, i.e. in the direction of "you're not likely to run out of fuel when you think this way." If you're really on a mission-critical fuel-critical run, you need to have lots of information and very careful planning. In fact, you're probably on a Green Beret on a life-or-death mission behind enemy lines in some remote part of the world and your logistics and operations teams have been supporting you through this process. If you're just out camping for a few weeks, you may have and feel justified in giving yourself quite a bit more slack.)
- There are a few other considerations about useable fuel. Did you run out of fuel on level ground? Or going uphill? Or downhill? Or on a traverse angle? All of these factors may make a difference in terms of how much fuel is really available for use by the engine.
- Thus, it's very important to leave yourself plenty of room for miscalculation and mishap when deciding how much fuel you need for a given trip segment, especially if the segment is likely to be fuel-critical.
Keep the fuel pump cool?
- Another thought about fuel reserves: If the fuel pump is in the tank (and it typically is on our rigs), you may wish to always have enough fuel on board to know that you're keeping the pump covered so it stays cool and therefore less prone to failure. But this should be taken care of based on our overly-cautious fuel planning, right?
Is your tank really full?
- Oh and how full was your tank to start with? Many of our rigs are unreliable about filling to the very top and we don't get to visually confirm that the tank is full to the top. In addition the fuel gauge may indicate full long before the tank is truly full, making it hard to know if you've taken on the amount of fuel you think you have.
When you are asking yourself whether you have a half-tank of fuel left, you'll start to know whether what you mean by half-a-tank is a fuel-gauge event or whether it is a how-much-fuel-did-I-really-burn and how-much-do-I-really-have-in-my-tank situation.
And that depends on useable fuel, right? Not just how much fits in your tank under optimal conditions, but how much was in your tank the last time you filled it.
Leaving the tank full when you're home
Leaving the tank full when you're home: Best for fuel condition, especially diesel regarding water and algae, etc. Best for emergency preparedness, including the proverbial zombie apocalypse. But not so good for gasoline, which has a short shelf-life. It can start going bad in as little as 3 weeks, but certainly by 3 months is getting questionable. That's why you should always use a fuel conditioner, e.g. Stabil or PRI-G (gas) or PRI-D (diesel) or something like those, when you're going to store your rig.
Hand-calculating fuel burn vs ScanGauge fuel burn numbers
I don't know how accurate the van's fuel metering system is, though I tend to believe that the ScanGauge is really reading what the van's computer says. The question is whether the computer is right. If it's really accurate, great. If it's not, I'd rather not find out the hard way. The only good way to know is to test. And it's hard to test with a high degree of precision. So once again, leave plenty of slack (i.e. margin for error) in your calculations and in your fuel-planning.
The reality is that constant monitoring of all the ways of knowing about your fuel burn, fuel availability, etc, is the best way to plan trips with tight-ish fuel requirements.
What about carrying extra jerry cans of fuel
The primary reason for carrying extra cans of fuel is when your tank capacity is too low for your predicted needs based on good fuel planning practices, i.e. you might still want or need to go to places where there is no fuel which are farther than your built-in tank will take you.
Or you might feel you need extra cans of fuel as insurance in case of fuel loss due to mechanical failure/breakage and you leaked all of your fuel in an environmentally sensitive area and wanted to get as far away as possible before your transgression was discovered by a roving gang of militant environmentalists.
If the latter is the case, you need to have a complete plan for utilizing your outboard fuel source.
FWIW, I've never had fuel tank damage off-road even when going extremely off-road in a passenger car, like the time I drove a BMW to Racetrack Dry Lake in Death Valley.
When to put the extra jerry cans of fuel in your main tank
So you've planned your trip and know that on this segment you're going to need the fuel in your tank plus 10 extra gallons and you've got those 10 gallons in jerry cans. Do you wait until the tank is empty or near-empty before using the fuel in the jerry cans? I don't think so. But there are mitigating factors, as usual.
If you think there is a real possibility that you will damage your fuel tank and will need the fuel to stay in the jerry cans (and have a plan for how to deal with the damage, etc) then maybe you should keep the fuel in the jerry cans until needed. Or if you think somebody else in your party will need fuel from your jerry cans, you may wish to keep the fuel in the jerry cans until needed.
But realistically, since you aren't going to carry jerry cans full of fuel unless your planning indicates you'll need them, and the other members of your party have planned their fuel adequately and you've discussed this as needed with them so everybody is comfortable with each others' planning abilities, and since the odds of breaking your van's fuel tank are pretty darn low and the odds of damaging your jerry cans and losing the fuel, while low, is still higher than damaging your main fuel system, you should pour the fuel from the jerry cans into your main tank as soon as it is convenient once you have burned enough fuel from your main tank to make room for the fuel coming from the jerry cans.
Once the jerry cans are empty, they travel more easily, they won't jounce fuel all over the back or top of your rig when the cap fails or fails to seal or comes loose, and they're no longer an easy way for somebody to steal fuel from you in a nice convenient package.
The kind of detailed planning illustrated above is overkill for most legs of most trips. Of course.
But planning fuel-critical missions requires a complete understanding of how much fuel you're likely to burn, plus reserves, plus a knowledge of how much fuel you can really drink from your tank.
This diatribe provided some tools for thinking about it.
And if you see me alongside the road with my thumb out, holding up a gas can, please give me a ride to the nearest gas station and back to my van.