Larger tires vs. Allison

neil · September 7, 2002

I have read several posts regarding an increase in tire size for their HD's. However, I came across the following in the Allison Transmission site:

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.

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.

I was wondering if anyone out there has experienced the side effects mentioned above after going to a larger tire size?

Thanks for your replies,

Neil

bluenote · September 7, 2002

This was recently discussed (not for the first time) in the forums at The Diesel Page. Many of these owners are running 255, 265, 285 and even 305 tires on their Allison equipped trucks. Most have reported no problems, even when towing. Several with the higher 200 # tires felt that they are getting better shifting with the replacment tires vs. the stock 245's.

TxDoc · September 8, 2002

You may want to search the Diesel Page and ask your dealer prior to switching about the info in this TSB below.

Apparently, the HD with the D/A combo will not be recalibrated by dealers (may depend upon who you know and who you are). There is a TSB regarding this. Here is some info on other's responses:

"Yesterday I had my truck (2001 2500HD 8.1/Allison) in to try and get an updated calibration for the Allison which they did take care of. Today the service manager calls me and said they just received a bulletin from GM that states that any truck with the Allsion transmission that has tires larger than stock (mine has 265-75-16) will have the warranty voided for the transmission. I informed him that my truck was purchased directly from them with these tires on them. They send trucks off to a custom shop to have certain upgrades done. They said they are informaing all customers that bought trucks with the upgraded tires to bring them back and have the original size tires put back on at the dealers cost. I told them that this was no deal because I bought this truck with these tires because I tow a trailer off road on a regualar basis and need the more aggressive tire. I would have never bought the truck if I could not upgrade them. At that point he said that he would consult with some other management there at the dealer and get back with me. WOW! I really don't know what to think at this point."

The TSB:

Info - Inspection of Tire and Wheel Size Prior to Diagnosis of

Transmission Shifts, Poor Performance, Speedometer, Cruise

Control Concerns #01-03-10-010

Inspection of Tire and Wheel Size Prior to Diagnosis of Transmission Shifts, Poor Performance, Speedometer, Cruise Control Concerns

2001-2002 Chevrolet and GMC C/K Pickup Models (Silverado, Sierra)

with Duramax? Diesel Engine and Allison® Transmission

Proper diagnosis of engine, transmission, speedometer, and cruise control concerns must begin with an inspection of tire/wheel size.

The 2500 series trucks have LT245/85R16E tires as standard equipment.

The 3500 series trucks have LT215/85R16D tires as standard equipment.

A truck that has different tires and wheels will have Engine power/performance concerns, Transmission shift concerns, Speedometer registered speed concerns, and Cruise Control concerns.

Before any diagnosis is performed for these types of concerns, inspect the tires/wheels for the proper tire and wheel combinations.

If a truck is found to have other than the above listed tire sizes, no further diagnosis should be undertaken until the original size tires and wheels are reinstalled

on the truck.

The Engine Control Unit (ECM) and Transmission Control Module (TCM) use (RPM) and (VSS) inputs as operational parameters for engine torque,

transmission shifting, cruise control operation and speedometer indicated vehicle speed.

Calibrations for different tire/wheel combinations are not available.

Therefore changing of tire and/or wheel size should not be undertaken by dealerships or truck owners.

Warranty Information

Reinstalling the proper wheels and tires on the vehicle, prior to performing any type of diagnosis of the truck, would be considered truck owner responsibility

and not a warranty repair.

SoMnDMAX · September 9, 2002

I've got 17,000+ miles on my truck with the 285's installed. I've pulled heavy, pulled light, empty, raced, truck pulls, etc, and I've never had any Allison issues that the OEM reflashes haven't fixed.

I personally think GM's bulletin is BS. Just how exactly does the TCM deduct actual ground speed from output shaft RPM on the Allison when there's input shaft and output shaft speed sensors on the Allison, and sensors at each wheel for the ABS?? There isn't a sensor for actual ground speed. The system should operate the same way, only with speedo error (indicated vs. actual), and possible ABS problems (easily fixed with a TECH II scan tool).

Some people pulling HEAVY could probably see some problems with overall performance due to a reduction in gear ratio. Most of those pulling that heavy shouldn't be pulling with 285's anyway, and should have load range "E" tires on their trucks.