GMC AWD diffrent tires front and rear

I would like to know if having 40 inch tires in the rear and 38 tires in the front is risky. It is awd and some say might damage the transfer case. I know with audis and subarus is risky, however the gmc gears are less precise so wondering if there is a percent tolerance. 2 inches out of 40 5% or is it 2 inch difference out of 38?

I don’t understand this. What measurement exactly is 40 or 38 inches?

I’m assuming you’re talking about tire diameter. A 2 inch difference in tire diameter is far to great for any AWD system to accommodate. However, you may or may not have a legit AWD system. The automatic 4WD system offered in some trucks is not a true AWD system, as they tend to engage the front axle automatically, but are incapable of shuffling differing amounts of power to and from the front and rear axles. The split is always 50/50, but the front drivetrain doesn’t engage until the rear wheels slip. At any rate it’s pretty dumb to using differing tire diameters on AWD or 4WD vehicles. If you have a center diff (AWD) you’ll burn it out in no time. If you have a traditional transfer case (4WD) you’ll wreck it as well, as they have next to no tolerance for different sized tires.


Sounds kind of vague to me, apparently like GM ring gears.

Yep, too great a difference, match them front to rear.

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the tires i purchased are sold with the measurements of diameter and width. not normal but you can look them up. they have a diameter of 38 the others 40. width are 15 inches when looking from front of vehicle.

yes, tire diameter. when you say 2 inches, its relative. these tires are about twice the size of the subaru that i used as an example. the off road sites use percentages of one tire as measurement base. Also, the split in awd is not always 50 50. some can do 70 30

what if the tires were a 100 inches across. its relative.

Why did you purchase 2 different size tire in the first place ? Also you do not need to reply to each individual as all of your posts are seen by everyone . What are these tires and why do you need them ?

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I was referring to the “automatic 4WD” systems that are common on GMC trucks/truck based SUVs. I was not referring to legitimate AWD systems. Some AWD systems can go 0/100 or 100/0 and everything in-between. The aforementioned automatic 4WD systems can only do a 50/50 split, as they don’t have a center diff.

??? I guess, but 38 vs 40 is too big a difference. Period.

Bat Signal to @CapriRacer - Do you want to weigh in here ?

Just read the owner’s manual. I’m betting it says to use same size wheels/tires or equipment damage can result. That means tolerance expected for same size not completely different wheel diameters. Add: mystery year but looked up make model- that’s what it says in plain english.

Why do people get combative when don’t get the answer they wanted?

Why do people get combative ? Because here in the US we now live in an ( Everyone gets a participation Trophy ) environment .

Here’s what Tire Rack has to say about mixing tires of different diameter. Cliff Notes version: don’t do it.


i am fine. but when people use words like dumb, i have dig in a little. i.e. FoDaddy above not appreciated.

long story short, the place that sold the tires and installed said very clearly it is fine. I am looking at legal avenues now.

I don’t see where anyone called you dumb . Forget legal because that will just cost you money for zero result. Just return to the shop and tell them you are not comfortable with 2 different size tires . That cost will be a lot less then what an Attorney will charge you for the first hour.

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you advice is appreciated however off. in one of the posts my actions were stated as dumb. i noted whom did it so you will have to search a little bit. not my fault you can’t see.

secondly, if any damage was done i would use legal recourse as my real profession is an attorney. So your estimate on how much an Attorney will cost is off a little (100%). And lastly, why would i go back to a shop does not look out for my best interests? Each tire is 600 dollars and i don’t plan on giving them any more of my trust nor business.

I think you’re being overly sensitive here. If it was the shop that insisted on the mismatched tires, then the statement seems to indicate that they did a dumb thing. Not you.

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