Fiji ST Posted June 21, 2018 Report Share Posted June 21, 2018 A buddy of mine just picked up a couple year old SUV that has computer-controlled AWD (no buttons/dials). The tires on it are all the same size and make, but they have slightly different tread depths. Now he is worried because he's always been told that the tires all have to be the same or it will throw off/damage the system. Is that even true anymore? I remember hearing about it back in the day, but thought I'd ask since I wasn't able to give him a definite answer. Quote Link to comment Share on other sites More sharing options...
iwashmycar Posted June 21, 2018 Report Share Posted June 21, 2018 I dont think it will make a difference. When turning they will spin differently...different air pressures would be a similar fate. I guess it could pull ever so slightly on a dead straight flat road... I dont feel like tread depth would make much difference unless its blatant...like full tread vs. zero tread. Some TPMS systems read differences in wheel speed to determine a low tire. Quote Link to comment Share on other sites More sharing options...
Geeto67 Posted June 21, 2018 Report Share Posted June 21, 2018 overall diameter "should" be roughly the same size for an AWD car, but there is some tolerance. BMW's factory AWD cars with 18" rims in staggered sizes has an estimated .4% difference in diameter from the factory. The logic is that the difference in tire sizes will stress the transfer case or transmission and eventually lead to failure. In some cases like subaru this is to true it's almost a religion, in others it's more of a semi-strict guideline. What kind of car is it? Each mfg has it's own tread depth measure tolerance for it's cars and you usually can find that info in the service manual. Generally speaking on a subaru all tires have to have a circumference within 1/4" (2/32nd tread depth) of each other, audi's run a little looser at 1/2" (4/32nd tread depth). It sounds like the car has a part time AWD system and it's either FWD or RWD unless there is a loss of traction, and someone hasn't been rotating the tires. He can try rotating the tires with the most tread to the drive wheels and check it until it balances out. he should also check that all tires are the same pressure since pressure affects tire diameter and tread depth. Quote Link to comment Share on other sites More sharing options...
Fiji ST Posted June 21, 2018 Author Report Share Posted June 21, 2018 overall diameter "should" be roughly the same size for an AWD car, but there is some tolerance. BMW's factory AWD cars with 18" rims in staggered sizes has an estimated .4% difference in diameter from the factory. The logic is that the difference in tire sizes will stress the transfer case or transmission and eventually lead to failure. In some cases like subaru this is to true it's almost a religion, in others it's more of a semi-strict guideline. What kind of car is it? Each mfg has it's own tread depth measure tolerance for it's cars and you usually can find that info in the service manual. Generally speaking on a subaru all tires have to have a circumference within 1/4" (2/32nd tread depth) of each other, audi's run a little looser at 1/2" (4/32nd tread depth). It sounds like the car has a part time AWD system and it's either FWD or RWD unless there is a loss of traction, and someone hasn't been rotating the tires. He can try rotating the tires with the most tread to the drive wheels and check it until it balances out. he should also check that all tires are the same pressure since pressure affects tire diameter and tread depth. Ford Edge. I believe one tire was brand new, the rest were around 6-8/32nds, according to his text. Quote Link to comment Share on other sites More sharing options...
Geeto67 Posted June 21, 2018 Report Share Posted June 21, 2018 Ford Edge. I believe one tire was brand new, the rest were around 6-8/32nds, according to his text. I keep seeing a reference to 3/32nds difference being the tread depth on an edge online, but don't quote me on that. If he's really worried, he could have the new tire shaved to match the others. Quote Link to comment Share on other sites More sharing options...
Fiji ST Posted June 21, 2018 Author Report Share Posted June 21, 2018 I keep seeing a reference to 3/32nds difference being the tread depth on an edge online, but don't quote me on that. If he's really worried, he could have the new tire shaved to match the others. I think I found the same information as you. I got him calmed down I think. This was just something I haven't heard about this in forever and wondered if it was still a true statement or not. Quote Link to comment Share on other sites More sharing options...
TTQ B4U Posted June 21, 2018 Report Share Posted June 21, 2018 I would think if there was an issue something within the systems of his car would be flashing a light and notifying him. No lights or warnings, likely no issues. Quote Link to comment Share on other sites More sharing options...
Cordell Posted June 21, 2018 Report Share Posted June 21, 2018 Yes it does matter, but generally tires that are close will not cause an issue. My training all comes from General Motors and they have a somewhat over thought out process to measure rollout. The 3/32nds rule is pretty good in most cases, you really need an extreme difference in wear to cause damage in most all wheel drive vehicles. However I can tell you Subaru is probably the most sensitive, they take the "symmetrical" in symmetrical all wheel drive seriously. For the record I have fixed a few AWD issues because some idiot just put 2 new tires on a vehicle even though the other 2 were worn out too. Thus causing a big difference in rollout, which is the real reason any of this matters. Quote Link to comment Share on other sites More sharing options...
Cordell Posted June 21, 2018 Report Share Posted June 21, 2018 I would think if there was an issue something within the systems of his car would be flashing a light and notifying him. No lights or warnings, likely no issues. That depends on the vehicle in question, I can tell you GM won't flash anything in most of its full time AWD systems, but it will bind and cause damage. These systems I'm refering to have no way to disengage so they aren't "smart" enough to know of any issues. Even like a Traverse with part time AWD won't warn you it'll just burn up the clutch at the rear diff. Quote Link to comment Share on other sites More sharing options...
STEVE-O Posted June 21, 2018 Report Share Posted June 21, 2018 good rule is 4/32nds difference max per most manufactures. I can tell you I have seen people not worry about it and put 2 new tires only on their subaru then get really pissed that the transfer case started binding up. Quote Link to comment Share on other sites More sharing options...
cstmg8 Posted June 22, 2018 Report Share Posted June 22, 2018 Am I the only one struggling with the math above??? 2/32" would be a 1/16" depth, not 1/4", also 4/32" would be 1/8", not 1/2". Correct? Sent from my Pixel 2 XL using Tapatalk Quote Link to comment Share on other sites More sharing options...
SpaceGhost Posted June 22, 2018 Report Share Posted June 22, 2018 We used tirerack for a replacement tire on a Subaru but we had tirerack shave the tire to the desired depth. No issue. Quote Link to comment Share on other sites More sharing options...
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