Tire Size Calibration

[countrymac]

Okay, here is a stupid question for someone.

I would like to use the Diablo Sport Predator to program my 2005 Sierra 2500 to know what size tires I have so hopefully the speedometer is accurate and the MPG is accurate and the amount of gallons consumed is accurate.

Currently everything is off...

I did some searching to see if I could figure this out myself but no luck.
The truck came factory with 245/75/R16
Currently the truck has....... 285/75/R16

Any help on giving me a dummies guide to programming the tire size would be most appreciated.

I'm really looking to get the best MPG with occasional towing of a trailer less then 10,000lbs.

[mikel]

Simple, plug in the factiry tire size then the new tire size to the Predator, then install the tune,and voila, speedo is correct.
Now, as for the DIC reporting properly, well, that may always be questionable.

Thanks

[countrymac]

Thanks Mike for your response. I would of tried doing just that but I downloaded the tire_conversion.xls and got totally lost.

Just to make things clear (so I don't assume) are you saying the DIC won't be accurate after changing the tire size? Or was that more a sarcastic humor in saying that the DIC should calculate correctly per the now updated tire size, but don't rely so heavily on the DIC to be accurate (which would explain why I saw so many remarks in here about hand/manually calculating milage).

Thanks again,
Dennis

[cowdoc]

country,
You will probably need to use a gps unit to accuratley adjust for your tires. For example, I am running 265/75/16's on my truck. I had to plug in 244/75/16 to get it accurate according to the GPS. Correcting for tire size will get your odometer correct, but if you are running anything besides stock tuning your DIC will not be accurate for mpg's or gallons of fuel used.

[mikel]

Yeah, the DIC is notoriously incorrect even in stock form, dont put too much faith in it.

Thanks