mismatched tires on AWD?

So, i had a blowout on my Subaru and against the tire shop's advice, only replaced the one tire. The set of tires was new 7 months ago and had only worn a few 32's. The tire guy assured my my transmission would blow up. Who's the idiot? Me or Him?

Just out of curiosity, I compared the circumference of the new tire to an old one and there is a 1/2 difference (82"-82.5" to be exact). I decided to put the new one on the rear and think my diff should be able to handle it. If it had any sort of LSD I'd be a bit more concerned, but I think it'll be ok. What's the verdict here? Do I drop ~$400 for 3 more tires?

I replaced two tires at once on my awd explorer about 15k(?) Ago. I went w/ a good set of used tires and kept them on the truck until I could replace the whole set. There was maybe 2/32s diff and I haven't had tranny issues yet. Im not a automotive professional though.

if you think about it... a bad alighnment can cause that kind of discrepency between tyres on the same axle

the tire shop put 3 205s and one 215 on my dads subaru, and no one noticed until they were worn out 100k later. It doesnt seem to have hurt it.

I had a flat due to a staple just outside the tread on a forester, probably about 3-4k miles after buying the car new.

The tire shop I went to gave me the same line about all 4 needing to be replaced. I had them swap the full size spare to the alloy and put the bad one on the steel spare.

I traded that in at 120k miles and had no transmission or differential problems.

Tire pressure could cause the effective diameter of the tires as much or more than you are seeing with the replacement.

If you are still concerned about it, rotate the tires more frequently to even out any imbalance in wear in the driveline.

I call BS on the tire shop. Their theory is that the new tire will turn at a different speed than the other tires and cause catastrophic driveline damage. This may be true if the vehicle was designed to move only in a straight line and had no tolerance for any slip in the system.

All cars make turns, which means that the inside tires and the outside tires move at different rotational speeds in every turn. The driveline is designed for this and it's not a problem. You will be fine with such a small difference in tire diameter.

It really depends on the rolling diameter. My only experience with it is a guy who had put two new tires on one end of his V70XC, then loaded his car full to move, and hit the highway. A thousand highway miles later his rear diff was burnt to a crisp.

In the tire shops defense- They will absolutely recommend changing all 4 tires on an all wheel drive vehicle. If a customer comes back in 3 months with a diff or transfer case bill from the dealership where warranty was declined because the car had mis matched tires, small claims court is making him pay the bill. As a shop owner, selling 4 is the prudent thing to do.

Streetwise said it first. Part of this is litigation protection, a vehicle can come in with mismatched tires after tens of thousands of miles for 2 more mismatched tires, blow out something in the driveline 5K later, guess who's on the legal hook? The last shop that put tires on. No matter how many waviers and warnings are signed and given unless that shop sells you 4 tires or refuses to service the vehicle, they're liable.

There is a technical aspect to this, different tread compounds do grip differently even on road tires. But so long as its the same exact tread compound, and the worst tread depth is no worse than 4/32" than the new tire it should be fine. I want to say I've even heard of a few shops offering tire shaving services so they can properly match up a single tire replacement.

So will your AWD system explode if you replace less than 4? Probably not, but if something does go wrong don't blame the place that tried to warn you. They're trying to cover your behind and their own too.

On most cars today, there's a certain percentage that you stay within depending on the center diff.

In your case, i wouldn't worry about it.

As long as all differentials work, there should be very little problem. Now a locked differential or a 4wd without a center differential is a whole other thing. Had a customer with a Jeep pickup. Complained that everytime he put it in 4wd it would lose power. Turns out, he had bald tires on the front and brand new tires on the rear, I told him to put the new tires on one side until he could get two more new tires.

Streetwiseguy wrote: It really depends on the rolling diameter. My only experience with it is a guy who had put two new tires on one end of his V70XC, then loaded his car full to move, and hit the highway. A thousand highway miles later his rear diff was burnt to a crisp.

Volvos are notorious for this, something about how their viscous diffs work. Which reminds me that I need to check the tire pressures on the wife's XC90...

Any kind of limited slip would be a NO-NO for different tire sizes. I don't see, if the tires were the same, why it would cause a problem. Now if it is AWD then the center diff might not like it.

AWD Mazda I have has warning stickers all over warning of ensuring all tires are same size and tread depth.

pigeon wrote:

Streetwiseguy wrote: It really depends on the rolling diameter. My only experience with it is a guy who had put two new tires on one end of his V70XC, then loaded his car full to move, and hit the highway. A thousand highway miles later his rear diff was burnt to a crisp.

Volvos are notorious for this, something about how their viscous diffs work. Which reminds me that I need to check the tire pressures on the wife's XC90...

The older ones- pre 2k I think, used a viscous drive. The newer ones use a pump and control unit to apply clutches. They are less friendly to rolling diameter problems.

pigeon wrote:

Streetwiseguy wrote: It really depends on the rolling diameter. My only experience with it is a guy who had put two new tires on one end of his V70XC, then loaded his car full to move, and hit the highway. A thousand highway miles later his rear diff was burnt to a crisp.

Volvos are notorious for this, something about how their viscous diffs work. Which reminds me that I need to check the tire pressures on the wife's XC90...

Something similar happened to my BIL. He had a V70XC, and put a pair used tires on the front that were labeled as the same size as the rears, but were a different brand and not really that close in size. It was a very expensive repair. On the bright side, that cured my wife from wanting a Volvo XC.

I work around semis and this is a big no-no. Four thirty-seconds is enough between sides to cause a problem. Obviously duals will wear each other out but the right set will wear the inside left out if they are deeper and the outside left out if they are shallower.

Not something that happened at any of my shops but I have seen a truck come in with a burnt diff with worn tires on one side and new tires on the other.

HappyAndy wrote:

pigeon wrote:

Streetwiseguy wrote: It really depends on the rolling diameter. My only experience with it is a guy who had put two new tires on one end of his V70XC, then loaded his car full to move, and hit the highway. A thousand highway miles later his rear diff was burnt to a crisp.

Volvos are notorious for this, something about how their viscous diffs work. Which reminds me that I need to check the tire pressures on the wife's XC90...

Something similar happened to my BIL. He had a V70XC, and put a pair used tires on the front that were labeled as the same size as the rears, but were a different brand and not really that close in size. It was a very expensive repair. On the bright side, that cured my wife from wanting a Volvo XC.

"used tires" !

I'm not sure I buy it- I had the same hard sell when I blew the sidewall of a tire during a rallycross. Not buying four E36 M3 tires from a shop on a sunday. Inflation pressure would probably make a bigger difference. I'd steer clear of any car that it would make a difference. By the way, the Subaru has a space saver spare, which makes no sense if tire size is that big a deal.

it's funny you mention space saver. I recently weighed the space saver in my saab versus the aluminum wheels that are on the car. There is not that much in the way of weight savings

I know for a fact it will confuse the hell out of the ABS on a Range Rover due to differing wheel speeds. Probably others as well. I'm sure the electronics were calibrated to act up before any damage occurred. As for the differential wear I think it depends on a lot of other factors. Design, age, maintenance history-it all adds up. If it was already weak for some reason then this could be wait finally causes it to fail...or it could last forever.

Now I'm as cheep as the next guy, but tires are a whole lot cheaper and easier to replace than differentials.

Teh E36 M3 wrote: I'm not sure I buy it- I had the same hard sell when I blew the sidewall of a tire during a rallycross. Not buying four E36 M3 tires from a shop on a sunday. Inflation pressure would probably make a bigger difference. I'd steer clear of any car that it would make a difference. By the way, the Subaru has a space saver spare, which makes no sense if tire size is that big a deal.

Space savers are skinnier, not shorter. Compare the diameter with the stock wheel and tire and they should be the same.

mad_machine wrote: it's funny you mention space saver. I recently weighed the space saver in my saab versus the aluminum wheels that are on the car. There is not that much in the way of weight savings

Thats why they don't call it a weight saver

Junkyard_Dog wrote:

Teh E36 M3 wrote: I'm not sure I buy it- I had the same hard sell when I blew the sidewall of a tire during a rallycross. Not buying four E36 M3 tires from a shop on a sunday. Inflation pressure would probably make a bigger difference. I'd steer clear of any car that it would make a difference. By the way, the Subaru has a space saver spare, which makes no sense if tire size is that big a deal.

Space savers are skinnier, not shorter. Compare the diameter with the stock wheel and tire and they should be the same.

Or the bigger issue of miles on the different tire. Space savers are supposed to be 50 miles max. I think you'd have to do several hundred miles to do any real damage.

Junkyard_Dog wrote:

Teh E36 M3 wrote: I'm not sure I buy it- I had the same hard sell when I blew the sidewall of a tire during a rallycross. Not buying four E36 M3 tires from a shop on a sunday. Inflation pressure would probably make a bigger difference. I'd steer clear of any car that it would make a difference. By the way, the Subaru has a space saver spare, which makes no sense if tire size is that big a deal.

Space savers are skinnier, not shorter. Compare the diameter with the stock wheel and tire and they should be the same.

I work at a big box tire store and we are told not to put one tire on a car with AWD but.. think about it if you have a space saver and you put it on your car with 45,000mi your stock tires should be just about bald even if the space saver was the exact diameter it would now be about 8/32nds to big I don't think any car manufactur would put anything in a car that would cause it to explode if you use it. just a thought

junkbuggie wrote:

Junkyard_Dog wrote:

Teh E36 M3 wrote: I'm not sure I buy it- I had the same hard sell when I blew the sidewall of a tire during a rallycross. Not buying four E36 M3 tires from a shop on a sunday. Inflation pressure would probably make a bigger difference. I'd steer clear of any car that it would make a difference. By the way, the Subaru has a space saver spare, which makes no sense if tire size is that big a deal.

Space savers are skinnier, not shorter. Compare the diameter with the stock wheel and tire and they should be the same.

I work at a big box tire store and we are told not to put one tire on a car with AWD but.. think about it if you have a space saver and you put it on your car with 45,000mi your stock tires should be just about bald even if the space saver was the exact diameter it would now be about 8/32nds to big I don't think any car manufactur would put anything in a car that would cause it to explode if you use it. just a thought

My Subaru manual says when you put on the space saver wheel to disable the FWD resistor to allow no power to be transferred to the rear diff. So maybe it's not that far from the truth. Of course it would vary from manufacturer.