I know in the thousands of threads I could find my answer, but I couldn't take it any more. Someone had actually put up a nice conversion tool for dodge. Had a red ram emblem as the background. One truck has 285's and the other 315's. I have yet to be able to calculate mpg

 

While I cant speak about or direct you to the thread in question but...
all you need to do is know the tire size that the speedo is calibrated for and the new tire size
ie original calibrated size 265/75/16 is 32 inches (however depending on specific truck config it could be 245)
285/75/16 is 33 inches
315/75/16 is 34.5 inches

33-32=1 /32=3% difference for a 285 tire over a 265

34.5-32=2.5 /32=7.8% difference for a 315 tire over a 265

multiply your mileage by 1.03 or 1.075 to use as your adjusted mileage in your mpg calculation

The above being said tire sizes vary to some degree for the same size tire one 265 might be listed as 32 inches and another one might be listed as 31.8. the more accurate the measurement the more accurate the calculation.

 

Thanks! Yea, my truck says stock tires are 235/85 r-16. The difference is 1.0915. Thanks again!