Final word on changing tire sizes...

[Diesel Duck]

My truck will be here any day & I really want to put larger & more aggressive tires on it. Not too aggressive or too large, but something that looks better than the stock tires.

I've read opinions here that say the dealership can recalibrate for the speedometer but that the Allison won't operate as intended.

What's the truth???

[Wally]

Unless it has changed in the last few months the dealer can not and will not recalibrate for larger tires. GM corporate controls the downloads necessary for tire size change and they will not issue them to the dealers. As far as the Allison goes larger tires will not noticeably affect its operation. Your speedo and odo will be off. Figure out how much and drive accordingly.
Your dealer may claim that he can calibrate for larger tires but all he can actually do is change the ABS portion of the program and this has no affect on the speedo or the tranny calibration.
There is a directive from GM that the dealers are not to investigate any low power claims or goofy shifting trannys if the customer has installed tires larger than OE. Some dealers use this to deny warranty work and other smarter dealers realize that if a truck drives funny it drives funny and larger tires didn't cause it.
Congratulations on the new ride!! You will enjoy it no matter what size wheels and tires you put on.

[Diesel Duck]

Wally,

Thanks for the info. I was afraid that would be the answer I would get. Now I'm hesitant to do anything with different sizes. It's too bad that GM doesn't give a new truck owner the option of a fairly agressive tire. I'm not going to be in the mud that often, but the street tires have just got to go.

[Bluerock]

Without incurring the cost of buying a new set of rims, you can easily put on a set of 255's with a good traction tread and then adjust your torsion bars to give you some added ground and tire clearance. Make sure to have the front end realigned after the torsion bar adjustment. I had a local front end shop do mine for $50.00 total (includes torsion adjustment and realignment). My speedo/odometer now reads about 3-4 mph on the slow side which is OK with me and the truck drives, rides, and shifts perfectly fine. Between the larger tires and the torsion adjustment the truck looks like truck should having that Monster - kid truck look. I also gained a couple inches in ground clearance under the transmission crossmember which was my main point of concern for getting high centered on a deep rutted road or hitting a rock. I think you'll like that new rig!

[SoCalDMAX]

Wally and Bluerock are both correct. I also have the 255/85 tires and love them. The truck looks a LOT better, but just as important, RPMs are 9% lower at every speed. I'd recommend one of 2 choices:

1. Driving it with the stock tires for 5,000mi or so to ensure you don't have any drivetrain concerns, then swapping them out for 255/85s if you want a 33" tire on the stock PY0 rims. This is what I did.

2. Buy some 16x8 rims and mount 285/75s. Keep the old setup and swap them back on whenever going back to the dealer. Pricier, but no dealer hassles. If I go into the dirt/sand in the future, I may be looking to buy 16x8s and 285s for the flotation.

on edit: I forgot about the speedo recal issue - the Superlift Truspeed will recal the signal for you. Or several ptogrammers will someday be released which will allow more power as well as speedo recal, shift points, etc.

Regards, Steve

[ 11-18-2002: Message edited by: SoCalDMAX ]</p>

[DMAXDiva]

SoCal described exactly what I did...put 285/75's on 16X8's and kept the OEM 245's on the stock rims for swapping anytime warranty work is necessary. So far, I haven't had to take the truck in for anything, but just in case, I'll do a quick swap (and remove the Juice of course!). I'm finding that the speedo is approximately 5mph off, so I just stick the cruise on 80 when I'm in a hurry! (hey, on Houston freeways, 85 is "flow-of-traffic"! )

Good luck with the new toy!

[Diesel Duck]

Thanks for the info guys! I'm anxious to get new tires that will look better than the stock ones!

[IHRAracer]

The 285's are the way to go, but I would use
16 x 10 wheels. Looks better to me with the right back spacing. What I have on mine.

[Zap]

Will 265x75x16 hurt the Allison or any other componet on the truck? If not why wont GM let you do it, they always did before, so there must be a reason.

[Ranger354]

I talked with a Gm service manager friend of mine and he said put whatever tires on the truck that I want. Then he will recalibrate it for me. $28 . He said that if something happened and they had to replace the transmittion or something then they would question the tires and NOT cover the work under warranty. but they will only question the tires if they are on the truck. I say that if something happens just swap them out.

He also said that GM comes out with computer upgrades for these truck all the time but never tell us about them.