Double tap

I had a 1997 Silverado 4x4 that was having issues shifting in/out of 4 wheel drive so I took it to a highly rated drivetrain service shop.
I had replaced 2 tires in the summer time and when winter time hit was when I first tried to use the 4wh drive.
2 new tires were on the front. After checking out everything I was told the difference in size of tires, newer vs older, while same brand & size were the problem.
The shop supervisor had a GM truck similar to mine he had the workers switch his tires onto my truck and told me to take a test drive. After 30 minutes of in/out 4wh drive testing I as convinced. Tires switched back to proper trucks, 2 new tires to replace older tires and everything was fined.
I was told AWD vehicles are worse to have that problem, so bad that they tell owners to replace all 4 tires before the will service the vehicles.[/QUOTE]

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Originally posted by egregore:

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So is this guy / business shooting for sales quotas, or can mismatched tires cause problems down the road...

If all the tires are fairly new, say, <20% worn, there is some wiggle room. In that case, replacing just the one tire shouldn't harm anything. If they are all significantly worn, one new tire, because its circumference is different, will cause problems with AWD differentials. The different rotation speeds will heat up the fluid or clutches (whatever it uses, if any) in the center and rear differentials and shorten their life. Similarly, you never use different sizes of tires on a RWD limited-slip differential. This is unfortunately the price you pay for AWD.

This is a good description and guide. I'd add that cars now with complicated stability control systems can get wonky with different circumference tires as well. I suspect you have such a system on a 2015 GMC.

You didn't post the mileage on the tires, but as others noted that's a relevant factor. I suspect they are originals, and have 30-40K on them?

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Originally posted by trapper189:

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Originally posted by mrbill345:
What does the owners manual say? Follow that recommendation.

I've got a 2013 Jeep Patriot & I had a tire dealer try to get me to replace all 4 tires saying because of the 4WD. My owners manual only recommends replacement in pairs. Needless to say that shop lost a sale & any future business.

4wd and Awd are different systems. I believe most Awds are controlled be computer as to which wheels to send power to. Having different size tires messes with the calculations. Most 4wds don't lock the front and rear driveshafts together in high range hence the
per axle requirement.

I know that. My initial point was to follow the manufacturer's recommendation.

My Jeep has Freedom Drive 1 which monitors wheel slippage & transfers power to the axle that needs it most. It also has the ability to engage & lock 4WD. With this system, Jeep recommends replacing tires in pairs.