How to tell mileage? - Diesel Forum

SmashedIxnay · 09-11-2005, 11:07 PM

I filled my truck up tonight and it was $108.50 @ 2.83 for 38 gallons. I don't know if my method is correct, so can someone tell me if I'm wrong?

I wrote down my mileage before I filled up, and next time I go to fill up I'm going to write it down, then divide it by 38 and that should roughly be my miles per gallon, right?

White 99 · 09-12-2005, 12:27 AM

Each time you fill up write the odometer miles number. Subtract the last fill up odometer miles number from the current number and divide the result by the gallons just purchased.

You want to know how many miles you drove since you last filled up and how many gallons it took to drive those miles.

I have a little note book that has all my fill ups along with the oil changes and the like.

If you really want to get good you can average the last two fill ups to remove filling errors. These trucks have a pretty good air bubble in the tank depending on how the tank is tilted while filling. Some spend the extra time to fill to the top of the filler tube but I think the air bubble is still in there causing inaccuracys. Best is to fill up at the same station, at the same pump, facing the same way and filling the same way; kind of a pain though. Or average to eliminate over filling and under filling errors. Don't believe anyone who gives you a great milage number but doesn't average the last two tank fulls.

SmokeyWren · 09-12-2005, 12:27 AM

Nah, that will be off by several percent. Because your tank holds more than 38 gallons, and you can't tell when it's full unless you spend about ten minutes and fill it until you see clear liquid at the top of the filler neck.

There is only one way to check fuel mileage:

Fill it up. Until you have liquid diesel - not foam - at the neck of the filler tube. This will take several minutes of dribbling it in. Then reset the tripmeter to zero. Then drive at least 300 miles, and preferably closer to 400 miles or more. Then fill it up again - to the brim again. Then divide the miles on the tripmeter by the gallons you filled it up with.

If your odometer/tripmeter is accurate, then that's your fuel mileage for the conditions you encountered on that tankful.

But most odometers are not accurate unless you have had the speedo calibrated for the odo to be right on the money.

Two ways to check your odo and determine the odo error.

Easiest is with a GPS. Remember to check distance, not speed. Compute the odometer error, then use that percentage to correct your odometer mileage when you compute fuel mileage.

The old fashioned way is to use the interstate mile markers. Find one that is not near an entry, exit, overpass, underpass, or any other reason the road crew could not place the marker exactly where it belongs. Put your front bumper on that marker and reset the tripmeter to zero. Then drive at least 10 miles without exiting the interstate or crossing a state line or changing highway numbers. The difference in your tripmeter mileage and the interstate mile marker mileage is the odometer error. Compute that as a percentage, then use that percentage to adjust your tripmeter mileage when computing fuel mileage.

If you use at least 10 miles on the interstate, that will get you within one percent of of the actual odo error. I usually try to use 100 miles, which gets me within one tenth of one percent.

White 99 · 09-12-2005, 12:38 AM

Good advice Smokey. [img]/ubbthreads/images/graemlins/smile.gif[/img]

SmashedIxnay · 09-12-2005, 12:58 AM

Well, when I was filling up it stopped at $101.49, but I got it all the way to $108.50 because some diesel started falling out about the $108 mark. I spent about 7 minutes from the $101.49 mark to the $108.50 mark. I think tomorrow I'll go and put whatever else I can spueeze into it and reset my trip mileage meter because I forgot to.

Thanks for the help guys.

djp9747 · 09-12-2005, 01:03 AM

I agree with Smokey's technique. The only RELIABLE way to calculate MPG is to fill up to the neck. Then take miles traveled divided by number of gallons put in. Also, don't just do this one time and take it as the gospel. Do this several times and average all the numbers. The reason is that your MPG can vary depending on a number of different reasons, ie type of diesel fuel, temperature, using A/C or not, % of time driving hwy / cty. You have no one to impress, so make sure you get realistic readings. And the only way to do that is to get several samples at different periods of the year over different types of terrain using different driving style.

I can't tell you how many people I know that say they have a 20, 21, 22 (or whatever) MPG truck, because on ONE trip, for a small segment of the trip, they were able to get that number. They will discard the fact that they normally average 15-17 MPG and tell everyone that they get 21 MPG in their truck.

SmashedIxnay · 09-12-2005, 01:09 AM

Anyone know what an average 2002 F350 Crew Cab dually gets for mpg? The only thing extra I have is a superchip, but I keep it on low settings and try to keep the truck under 2000rpm.