So what is the tried and true method of calculating MPG?

My intent is to figure out what MPG the truck is "really" getting (the on-board display shows ~16mpg).

The truck is a 2003 PSD with a Banks 6-Gun (set on "6"), intake & exhaust upgrades, 37" tires and 4.56 gears.

Thanks!

 

The way I do mine is: 1st, reset the trip meter, 2nd, fill the truck up to the neck. On refuel fill up to the neck and divide the miles by the number of gallons of fuel put in. Thats probably the closest you will get.

 

do the above for several fills to a month, add up all miles and gallons, THEN average. more coditions are thus accounted for and you'll get a more real world average.

 

It is very important that the speedo is correct so that you have the correct number
of miles to divide the gallons into. Like Spence said the more miles the better average.
Hwy mileage will be different than city or mixed mileage.
Because diesel foams, take your time and try to fill to the same spot in the neck everytime.

 

Ahh..... and since I know my speedo is off quite a bit (gearing & tires), I guess using the GPS is the next best thing.... correct?

 

i would suppose that GPS would be the next best. i normally average 15.5 to 16 in town, or 18 to 19 hwy, and around 12 towing. total average 17.1 total average over over 20K driven.

since i haven't done the harpoon to my tank, i guesstimate how many gal i'll put in it, and drop the fill nozzle down a setting or two for that last couple gallons, and then fill up to the next dollar (or the next dollar yet if it's, say, at .70 or .80). works well for me and it gets pretty accurate results tank to tank.

 

If your speedo is calibrated for a different tire size, you can multiply your trip odometer miles by the ratio of the actual tire circumference over the calibrated tire circumference to compensate. For example, my speed is calibrated for 245's, and I run 265's. So to compensate I could multiply my trip odometer miles by 99.43" / 95.72" (1.03876).

That is the method I usually use. However, this method doesn't consider tread wear and the constantly changing circumference of the tires. So when I really want to be particular, I reset the trip odometer in my GPS and go by that.

For filling, I do as mentioned previously with one additional detail. I always fill to the neck, but I always fill at the same place and park the same direction next to the same pump. If the truck is on any kind of slope, that will effect how much fuel it takes to fill the tank. Parking in the same place keeps this consistent.

When a trip forces me to fill up at a difference station, I wait to calculate my mileage until the next time I refuel at my usual place. Then I use the total miles since the last usual place fill-up and the total gallons burned in between usual place fill-ups to calculate my mileage.

I'm always looking for ways to improve the accuracy of mileage calculations, especially these days...