Someone help me with the math....MPG Thread
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Fleet Mechanic
Joined: Jul 2014
Posts: 1,727
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From: Almost back in TX biotche
Someone help me with the math....MPG Thread
I know we all try to figure out how to get better mileage. It is a daily struggle for many of us, others have accepted the fact that your MPG will never be great; and others, like Tom, go out and buy something that gets 50mpg for a nice blended average....
A while back I started filling the tank at different levels of empty each time to see if there was any difference. I have noticed that I get better fuel mileage when I drain an entire tank vs when I fill at the halfway point.
I will not bore you with all the details but the averages are this:
Full to Empty - Average 16.8mpg
Full to 1/2 - Average 16.1mpg
So if I am getting .7mpg better when draining the tank 100% vs only 50% can one assume that I am getting 1.4mpg better on the second half of the tank? The only variable that is really changing is weight. Because this was done over several months, factors like where I got the diesel and the temperature would not make a huge difference on the overall average. On times during towing, I would not record that data from those fill-ups since I figured it would mess with the numbers too much.
What are y'alls thoughts?
A while back I started filling the tank at different levels of empty each time to see if there was any difference. I have noticed that I get better fuel mileage when I drain an entire tank vs when I fill at the halfway point.
I will not bore you with all the details but the averages are this:
Full to Empty - Average 16.8mpg
Full to 1/2 - Average 16.1mpg
So if I am getting .7mpg better when draining the tank 100% vs only 50% can one assume that I am getting 1.4mpg better on the second half of the tank? The only variable that is really changing is weight. Because this was done over several months, factors like where I got the diesel and the temperature would not make a huge difference on the overall average. On times during towing, I would not record that data from those fill-ups since I figured it would mess with the numbers too much.
What are y'alls thoughts?
Post Fiend
Joined: Apr 2002
Posts: 19,311
Likes: 97
From: Waterloo, Iowa
When dealing with tenths it's necessary to be making accurate comparisons. Refuel at the same location, same fuel type, from the same pump, make sure the tank is topped off exactly the same level every time, just one "click" etc.
Big trucks have the aerodynamics of a brick so wind plays a big factor during your test. A full tank mpg check is going to be more accurate than over half a tank.
Big trucks have the aerodynamics of a brick so wind plays a big factor during your test. A full tank mpg check is going to be more accurate than over half a tank.
Post Fiend
Joined: Mar 2002
Posts: 19,402
Likes: 38
From: Bonita Springs FL
0.7 mpg is really not that much. There are a number of factors, most of which Ted listed, that impact mileage in addition to driving style, elevation changes, etc.
I would guess that you are within acceptable variance, and your observations are not statistically significant. The weight of the unburned fuel is a factor but a very tiny one and I can't think of any other reason why you'd see different mileage numbers based on the fullness of the tank.
If you were able to turn in mileage that was consistently 16.8 +/-0.1 on an empty tank and 16.1 +/- 0.1 on a half-empty tank, then that would be significant. But I know mileage records over a long period of time are never that consistent. You can see as much as 5 MPG difference from tank to tank depending on all those factors above, and the data just isn't going to be consistent enough to support the conclusion that tank fullness impacts mileage.
I would guess that you are within acceptable variance, and your observations are not statistically significant. The weight of the unburned fuel is a factor but a very tiny one and I can't think of any other reason why you'd see different mileage numbers based on the fullness of the tank.
If you were able to turn in mileage that was consistently 16.8 +/-0.1 on an empty tank and 16.1 +/- 0.1 on a half-empty tank, then that would be significant. But I know mileage records over a long period of time are never that consistent. You can see as much as 5 MPG difference from tank to tank depending on all those factors above, and the data just isn't going to be consistent enough to support the conclusion that tank fullness impacts mileage.
Post Fiend
Joined: Feb 2011
Posts: 26,410
Likes: 6
From: Ft. Lauderdale, FL
Diesel is about 7.1 lb per gallon so at half tank you have burned off 156 lbs approximately or the weight of a chunky wife. Theoretically you would get more mpg on the 1/2 tank to empty than full to half.
I can notice a huge difference on my boat when I'm down to half a tank (7100 lbs less) I see a 10-15 gallon per hour savings.
I can notice a huge difference on my boat when I'm down to half a tank (7100 lbs less) I see a 10-15 gallon per hour savings.
Post Fiend
Joined: Feb 2011
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From: Ft. Lauderdale, FL
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Lead Driver
Joined: Oct 2010
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From: Ouray, CO
When you fill up from nearly empty, you get more foam in the tank which results in less fuel going in. With half a tank, you get less foam resulting in more fuel going in.
Plus, your mpg varies during each commute, warming up compared to later in the commute. If you fillup on the way to work, you penalized the prior tank with the warm-up and the next tank benefits from a fully warmed up start. Did you stop for groceries, beer, etc. on this tank vs prior?
Things like this make it very difficult to make real comparisons, and a couple of WOT to get on the highway negate a day's worth of driving "boringly".
Plus, your mpg varies during each commute, warming up compared to later in the commute. If you fillup on the way to work, you penalized the prior tank with the warm-up and the next tank benefits from a fully warmed up start. Did you stop for groceries, beer, etc. on this tank vs prior?
Things like this make it very difficult to make real comparisons, and a couple of WOT to get on the highway negate a day's worth of driving "boringly".
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Thread Starter
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Fleet Mechanic
Joined: Jul 2014
Posts: 1,727
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From: Almost back in TX biotche
If you were able to turn in mileage that was consistently 16.8 +/-0.1 on an empty tank and 16.1 +/- 0.1 on a half-empty tank, then that would be significant. But I know mileage records over a long period of time are never that consistent. You can see as much as 5 MPG difference from tank to tank depending on all those factors above, and the data just isn't going to be consistent enough to support the conclusion that tank fullness impacts mileage.
Over the next 2 months I will be doing 100 miles a day as a commute. I may be able to make sure I always fill up at the same station and drive the exact same way. I don't see traffic much on that route so I should be able to run constant speeds. This will allow me to see if there really is a difference over a short period of time.
Diesel is about 7.1 lb per gallon so at half tank you have burned off 156 lbs approximately or the weight of a chunky wife. Theoretically you would get more mpg on the 1/2 tank to empty than full to half.
I can notice a huge difference on my boat when I'm down to half a tank (7100 lbs less) I see a 10-15 gallon per hour savings.
I can notice a huge difference on my boat when I'm down to half a tank (7100 lbs less) I see a 10-15 gallon per hour savings.
HAHA
I try to be fairly easy on the pedal. When not towing, I try to keep my RPMS under 2000. I try to keep my showing off to a minimum as well, but that is sometimes very hard to do.
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Stewart
Post Fiend
Joined: Mar 2015
Posts: 5,277
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From: Blairsville, Pennsylvania
I just put it on fastest setting and let it fly!
It will stop when the pre-auth limit of $100 is reached. That won't fill the tank here in PA, land of the "Tax 'em until they squeal"
I haven't seen a topped off tank since...what year is this?
It will stop when the pre-auth limit of $100 is reached. That won't fill the tank here in PA, land of the "Tax 'em until they squeal"
I haven't seen a topped off tank since...what year is this?