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Old 07-28-2012, 07:17 PM   #11
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A basic engine and mpg question

My 89 SMB has two tanks, one is 15 and the other is 18 gallons. There is one fuel gage with a manual transfer switch. I have gone 80+ miles with the gauge at the red line before draining the gas and having to switch tanks. At 14mpg that means there is about six gallons left in the tank at the red line.

Thanks for the spreadsheet.
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Old 07-28-2012, 07:33 PM   #12
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Re: A basic engine and mpg question

larrie,

Do I understand correctly that you actually run your tanks dry before switching?

And you get the same results with either tank?
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Old 07-28-2012, 08:31 PM   #13
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Re: A basic engine and mpg question

redrock,

Since your van has been converted to four wheel drive they would of had to do something with the tamk size. The stock tank will not clear the transfer case. If they cut the original tank down it is probably only 32 or 33 gallons now. I also have a 2008 V10 but its still for now two wheel drive so stock 35-37 gallon tank depending on where you read it. One night I was coming back from Portland and it was late and I just wanted to get home. From my calculations I was pretty sure I could make it home with the fuel I had left. After the low fuel light came on I went 82 miles more and made it home. The next day it ran out of gas as I made the first turn from my street not even 1/4 mile away 532 miles since my last refill. Now I know I can go at least 450-475 miles on a trip (only highway driving) but it will almost always be out of gas by 500-525 miles. The extra plus or minus 50 depends on how good or bad the fuel is or any other variations.
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Old 07-28-2012, 09:47 PM   #14
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A basic engine and mpg question

Quote:
Originally Posted by redrock
larrie,

Do I understand correctly that you actually run your tanks dry before switching?

And you get the same results with either tank?
Redrock,

Yes I did run each tank dry, but only once for each tank so I could figure out how much they held. Was concerned that the gauge was messed up because even thought they were suppose to be 15 and 18 gallons. When I filled them at the red line they only took about 9 and 13. The only way to figure out what was going on was to run them dry. By dry I mean when the engine started to sputter. yes I know all about the dangers of doing this but I need to know what was going on with the gauge.

Yes the results are about the same on either tank although I usually run on the 15 gallon tank first. When that gets to the red line I switch to the 18 gallon tank. That way I know I have a reserve if I am between gas station when the 18 get low. It is comforting to know that I can go about 80 miles more after the gauge is on the red line.
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Old 07-29-2012, 03:22 PM   #15
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Re: A basic engine and mpg question

Ultrasport,

The conversion was done by Advanced 4x4 Systems (http://advanced4x4vans.com/). I am pretty sure that the original tank is intact.


larrie and Ultrasport,

you guys are braver than I regarding running your tanks dry, but it does certainly give you an accurate usable fuel number. (Remembering of course that usable fuel may vary based on attitude, i.e. level ground, vs uphill/downhill/off-camber or a combination of all three.)

Maybe it would be worth it to run the machine to a sputter sometime. I'll consider it.

I have a plane with 2 main and 2 aux tanks and if I'm alone or with non-nervous passengers I prefer to run the aux tanks dry (to first sign of roughness, which is maybe 15 seconds before dry) before switching, only because the aux tanks are not authorized for use in takeoffs and landings and it's just not sane to keep 15 minutes of fuel in them. I prefer to know that I've emptied them out. I can then ensure how much fuel really went in when refueling. All that becomes part of the known fuel-use history and allows for better fuel management planning.

I'm trying to figure out if fuel-management is more or less critical off-roading vs flying. When flying, my personal minimum is a minimum of 1 hour of usable fuel in the tanks when I land. Period. (The law requires less - 30 minutes day VFR, 45 minutes night or IFR.) The plane's burn rates are very predictable and tunable for range as needed. The van, on the other hand, uses fuel very differently in different situations. Plus there is no certified usable-fuel amount, unlike the plane.

I want to always know how much fuel I have and how the machine drinks is going to drink it during my planned use, so I can figure out whether I really have enough fuel to execute my plan. I suppose in the end, though, a minimum of one hour of fuel onboard the van seems reasonable. Maybe I'd go a little less in the van because at least there's no off-airport landing possibility. To actually plan to have only two gallons on board the van at the next refueling seems like a nail-biter. That would only be 25 miles at highway speed or maybe 12 or 14 miles on rough terrain. I'm pretty sure I should always have more than that.

Maybe I'm just too obsessive about it.

But I don't figure I'm going to get very far pushing the van on foot.
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Old 07-29-2012, 03:48 PM   #16
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Re: A basic engine and mpg question

FWIW, with all my vehicles, I like to refuel when I'm down to no more than 100 miles left on the tank, especially my diesels ('01 Excursion and '05 Passat). In my opinion, fuel injected engines are very sensitive to contaminated fuel, and in my uneducated opinion, running a fuel tank low leads to an increased possibility of picking up contaminants. This is especially true of water. The more unoccupied volume in a tank, the more water vapor can enter and as it condenses, the water will settle to the bottom of the tank. I think most fuel pick ups "float" so they don't pick up the water. Less diesel - more likely to pick up water.

Then there's also sucking air into the fuel injection system if things get really dry in the tank. More bad juju.

Lastly, another reason I try to keep a minimum of 100 miles in the tank is in case of a disaster. Good luck fueling up if there's no power, or if everyone else in town has the same thought and isn't prepared. In this case, 100 miles is probably insufficient.


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Old 07-29-2012, 09:27 PM   #17
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Re: A basic engine and mpg question

I think the rule of thumb is to refuel by the time you're down to a half tank.

Sometimes, depending on routing and expectation of fuel availability, one might top off at every opportunity. There are definitely plenty of places I've been where the availability of fuel is uncertain. And a number of times I've had to plan backcountry routes where it was conceivable that I could get "almost" to the next fuel supply but not be sure that I could actually get through and also not be sure that the fuel supply would be available when I got there. Once I actually had retrace almost my entire route to get fuel due to difficulties on the way and the uncertainty of fuel availability if I continued. I wasn't sure if I could make it to an alternate source of fuel if the primary source proved to be closed or otherwise out of service.
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Old 07-29-2012, 10:50 PM   #18
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Re: A basic engine and mpg question

I didnt intend to run out. Its the very first time in almost 40 years of driving I ever ran out. I thought I had enough to get to the closest staion to my house...maybe a mile. All it did was sputter on a right hand corner and I knew it was on fumes. I shut it off and went for gas.

Since electric fuel pumps are cooled by the fuel its never good to try to run them when they are out. I usually leave about 100 miles to empty on trips and I usually get gas before the light comes on around home as well. There is something I dont like about paying more than $100 to fill the van. But I do let it run low from time to time. I am a big believer in regular fule filter changes. Any issues or potential problems it gets a filter.

Herb I agree completely that it is bad to run out of fuel. As a life long automotive technician and trainer I have seen many bad things from running out of fuel. My vans is a V-10 and with all the ethanol we have in CA I dont think water is very likely.

Diesels are different there are lots of potential problems if they get even the slightest bit of water, thats way they have such big fuel filters.
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Old 07-29-2012, 11:10 PM   #19
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Re: A basic engine and mpg question

After a trip, I always top up the diesel, propane and water. That's to keep water from condensing in the fuel tank and to keep the van ready to be a mobile disaster shelter if necessary.

On the road, I stop for fuel at about 1/2 tank or sooner depending on what's up ahead. I've never put more than 30 gal into what is supposed to be a 46 gal tank. My gauge stays pegged on full for about 100 miles after a fill up and I have no idea exactly how much usable fuel I have or where my gauge reads when it's out. Like others - I'm afraid to run it dry to see.

I tend to use the odometer as much as the fuel gauge. I figure a worst case 10mpg and 43 usable gals for a 430 mile range. I zero out the trip odometer when I fill up and start to get nervous when it gets near 400.
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Old 07-29-2012, 11:29 PM   #20
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Re: A basic engine and mpg question

When I added the transfer case to my '97 E350 EB with V10, I had to modify the fuel tank as everyone does. I cut off as little of the nose of the tank as I could get away with (left about an inch between the tank and the transfer case) and welded on a new end. Obviously the tank was completely empty, before welding on it, and I only added about 1/2 a gallon to the tank to drive it to the gas station down the street. I ran it out of gas at the station and then filled it up to the neck; took 35.1 gallons which surprised me because the owner's manual indicates the fuel capacity to be 35 gallons and I estimated the volume of the removed portion of the tank to be 2.5 - 3 gallons I then drove the van until the fuel gauge was in the red and filled it back up - 28 gallons so the "reserve" seems to be 5 - 7 gallons.
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