MPG calculations help needed

[The_Dealer]

trying to figure my mpg's since i got my carb tuned right and everything. now that everything is adjusted correctly, #61 jets are running too rich, gonna try some #58 and see. anyways i know how to calculate my mpg's, but heres the thing. my speedo is 3mph slower then what im actually going(from the tire calculator to convert 225's to inches) and from the mph actually traveled. it has been verified by gps. so how do i factor that into my actual mileage? do i just add 3mpg to my final calculation or is there more to it??

 

[strvger]

can't you just fill the tank, set the trip meter to zero, drive it to the half full mark, refill it, divide the gallons needed for the refill into the miles driven?

edit: do that a few times to get a pretty accurate reading.

 

[TheTopher]

First off you need to find a percentage difference, not just a definite 3mph.

After that you need to take your trip odometer reading and correct it with your speed difference. For instance, if your speedometer is 5% higher than before, you would take your miles driven and divide them by 0.95 If it's 5% lower than before, divide it by 1.05

Then you take that number and divide it by how many gallons your truck took.



If you want to post the before and after tire sizes and your miles driven, and the gallons your truck took I'd be happy to do the math for you.