I recalibrated my speedometer with a Tech 2. IF the tire size (diameter) is sanctioned for a similar truck model to your own you can probably do it by "lieing" to the Tech 2. Enter the other model/year when you are building the truck and you will be given the option of changing the tire size. This will deal effectively with the speedo, ECM and BCM but NOT the TCM (transmission).

OOps! I should have put this in the 2500/3500 forum. :o:

IF YOU HAVE THE ALLISON, DO NOT USE NON-STOCK TIRES! You can change the speedo and the odometer, but the TCM is not changeable. You will be feeding your transmission control module wrong data and mess up your shifting and this is no good for the Allison. Go to http://www.allisontransmission.com/service/faq/index.jsp?CategoryID=11#120 -

Q. Speedometer / Tires - Can my speedo be re-calibrated after I install larger tires?
A. Speedometer calibrations are only available for tire sizes offered as original equipment. Please consult your GM dealer about correcting speedometer output when tire size is changed.
NOTE: Changing the calibration to correct speedometer readout does not change the TCM calibration, and thus it does not compensate for the adverse impact on transmission operation.

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Q. Speedometer / Tires - What effects will big tires have on transmission operation?
A. The TCM calibration is based on factory tire size and rear axle ratio. Changing either of these parameters will have some adverse impact on transmission operation, including features like grade braking and shift stabilization. For instance, we have noted that when P285 tires are installed that the transmission generally does not shift to 5th range when towing heavy loads.

Dude et al -

I'm running 265x75's and never pull a trailer. I don't anticipate any problems. But if I were anticipating pulling a lot of miles with a heavy trailer I would certainly stick to stock tire sizes. Also, I would never run grossly oversize tires because of the additional stresses. But that's just me. Different folks, different strokes.

On the other had, even with my miniscule up-sizing, I will never go to the dealer crying for warranty work on anything from the tranny back that could be attributed to the oversize tires. Of course that would be inadvisable anyway since Mike L. has done a little fiddling around in my tranny. :)

It is my understanding that, because different sized tires will affect how the automatic transmission performs, the GM policy is to advise dealers to do no transmission warranty diagnosis or remediation until the owner replaces the tires/wheels with OEM size.

It does not "void" your warranty. You just can't get warranty work done until you come in with OEM size tires/wheels. For this reason, my winter set of tires are stock size on a spare set of PY0 wheels that I picked up. If I need warranty work on the Allison, I will change over to the 245 tires.

I think there is something about this in the service bulletin part of this web site.