Ok Guy's, these are the MODs i have done to my 03 CTD 2500 truck:


Went from 265/70/17 to 285/70/17..Added K&N Intake, 4" dual exha, and Quadzilla. My mileage before MODs 18mpg(Per Computer)
After MODS 24mpg. I know the computer is wrong because of the new diameter of the tires..I've seen it here before but can't find it now, could someone please tell me the calculation to hand calc my mpg taking into account the diameter difference from the tires?

Thanks,

Black 03 2500 CTD

 

I run 285-70-17's in the winter. I used a 4% difference in mileage calculations. I run the race box and my overhead reads about 2 mpg high over hand calculation.

 

There is no way to know how inaccurate the overhead mpg is with a performance box. The overhead is not necessarily accurate completely stock. Stock my overhead was .9-1.2 mpg low. The Quad Race on mine would read anywhere from 1.5-2.5 mpg high. Hand calculate the mileage to get a true mpg.

This tire calculator show Speedometer Difference: 2.028% too slow for your tire change.

http://www.1010tires.com/TireSizeCalculator.asp

 

just go to the dealer and give them 45$ and they will fix your speedometer
Your MPG reading on the overhead display will still be wrong, the quadzilla throws it off. My edge did the same thing, the overhead says 22-23 when i am getting 17-19.