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Old 09-26-2009, 06:23 PM   #1
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Better gas mileage

During the quiet drive to Alaska, I used my Scanguage to monitor gas mileage at different speeds. As a result I changed my driving habits, and so I thought I'd share my findings with the list. At 75 mph my 2006 v10 gasoline 4x4 SMB with kayak on the roof averaged 8 mpg! Slowing down to 55 mph saves a lot of gas ($$$), reducing consumption to 13.5 mpg. I could even get 15 mpg driving at 40mph, but slower than that and the gas mileage actually started to get worse. I am sure the results are not perfect, but I repeated these 10 minute tests multiple times, usually along a body of water to be certain it was flat. However, I wasn't able to control for wind.

When gasoline was at $4/gallon, slowing down to 55 mph is like getting paid $40 an hour!
Here's the calculation: 100 miles at 55mph will take me 1.82 hours and use 7.4 gallons, while at 75mph it will take me 1.33 hours and use 12.5 gallons. So driving 55 takes half an hour longer, but saves about 5 gallons (=$20). Thus, it's like getting paid $40/hour - better yet, I'd have to earn $70/hour to net the same amount after income taxes. Makes me feel pretty good to slow down and enjoy the scenery a bit more - hurrying is just too expensive.

-JR

Here's my data:
30 mph = 13.6 mpg
35 mph = 15.4 mpg
40 mph = 15.0 mpg
45 mph = 14.5 mpg
50 mph = 13.9 mpg
55 mph = 13.5 mpg
60 mph = 12.4 mpg
65 mph = 11.5 mpg
70 mph = 10.0 mpg
75 mph = 8.2 mpg
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Old 09-26-2009, 06:43 PM   #2
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Re: Better gas mileage

Wow, great info. When I drive my SMB I usually set the cruise at 65 or 75, whatever the speed limit is, and don't think too much about it. I bought a scangauge a couple of months back because I'm a gauge watcher and a bit of a nerd. But seriously to monitor Transmission Fluid Temp. I'm still in the process of getting it calibrated and haven't driven the SMB too much this summer due to my job getting in the way of my social life. The previous owner said that he usually got around 14-16 mpg but I haven't tested this. I'm down to about 10 gallons in my 58 gallon dual tanks so next fill up I should be set. I have the mighty 7.3 PSD but these numbers will be good to keep in mind. Thanks...
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Old 09-26-2009, 07:59 PM   #3
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Re: Better gas mileage

That kayak must be impacting your mileage since we, also with a V10 4WD, typically get around 15-16 mph at 60 mph via the Scangauge when on level terrain. Our mileage is actually pretty good compared to a Suburban but the problem is in accelerating....getting up to highway speeds results in mileage of from 4 mpg to 8 mpg.
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Old 09-26-2009, 10:08 PM   #4
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Re: Better gas mileage

Do you know what gear ratio you have in your axles and what size tires you use?
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Old 09-26-2009, 10:22 PM   #5
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Re: Better gas mileage

Quote:
Originally Posted by Stumpalump
Do you know what gear ratio you have in your axles and what size tires you use?
The tires are BFG 285/75 R16. Not sure about the gear ratio, but it is a 2006 model gasoline E350 with the SMB 4WD conversion.

-JR
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Old 09-26-2009, 10:29 PM   #6
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Re: Better gas mileage

The gear ratio you have can affect mileage, but usually not much. The power needed to go a given speed doesn't change and it takes about the same fuel to make the same power.

But, to clarify the gear ratio question, can you tell us how many RPM's you turn at 60mph? We can figure the gear ratio from that.

Mike
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Old 09-26-2009, 11:05 PM   #7
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Re: Better gas mileage

As much as I like the ScanGuage, I wish it would integrate the MPG over a somewhat longer period of time. It's so sensitive to any change in any number of variables, I swear my MPGs can fluctuate by 50% if a butterfly passes gas three counties away. Not quite the context Ray Bradbury and others have written about, but you get my point (maybe). I will say the mileage with my SMB with 6.0L turbo diesel is VERY dependent upon my cruising speed.

I can get 9999 MPG if I take my foot off the accelerator after a hard acceleration.


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Old 09-27-2009, 01:29 AM   #8
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Re: Better gas mileage

Quote:
Originally Posted by Bronco_hauler
As much as I like the ScanGuage, I wish it would integrate the MPG over a somewhat longer period of time. It's so sensitive to any change in any number of variables, I swear my MPGs can fluctuate by 50% if a butterfly passes gas three counties away. Not quite the context Ray Bradbury and others have written about, but you get my point (maybe). I will say the mileage with my SMB with 6.0L turbo diesel is VERY dependent upon my cruising speed.

I can get 9999 MPG if I take my foot off the accelerator after a hard acceleration.


Herb
There is an Average MPG feature that will integrate over any time you set. Just reset AVG and you'll get the average mileage since that point.
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Old 09-30-2009, 04:20 PM   #9
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Re: Better gas mileage

Interesting! I just returned from 2400 miles up and back from BC. I kept track and my AVG. mpg was 11.2 I don't have scan guage just an odometer and a calculator. I kept my speed at or below 60. How much does weight play in the calcs? I'm not a fully built SMB so I assume I weigh around 8500lbs. I run 285/75/17s with 52lbs of air. My odometer reads about 3% below my GPS.
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Old 09-30-2009, 08:23 PM   #10
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Re: Better gas mileage

By watching our Scangauge, it is obvious that what really impacts MPG is accelerating up to cruising speed. I'm down to probably 3 mpg (as I recall) in first gear and then about 8-9 mpg in second. But at cruising speed on level grade, we get 15-16 MPG as I mentioned. So, our average MPG would be relatively good if we could drive on a level freeway at 60 mph for 600 miles! It is the getting up to cruising speed that impacts our MPG tremendously and the hills!
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