I was at the tire shop today to have all 4 tires (Hankook Ventus V12s) replaced with Michelin Pilot Super Sports. Unfortunately, one of the new rear tires had a 3" long cut in the sidewall. NICE. Long story short, they installed 3 of the new PSS tires and I still have an older tire on with a some tread remaining. A new tire is on the way, but it will take about 4 days to arrive.

My question is, will this be a problem if I drive the car? I know traction is a small issue but I'm more concerned about any drivetrain issues, especially since my targa is AWD. I've been a long time Audi owner & I know that there can be some issues if there is too much diameter difference F/R but this has mainly been from people running a staggered set up.

My guess is it won't be an issue but wanted to consult the local brain trust here.....

 

Porsche puts a limit of 30% difference in tread depth for tires on the same axle.

If the old tire is worn more than 30% from a new tire on the other side then the old tire should not be used. The car should sit until the new tire arrives.

 

You mean 3 of them are MPSS, and one is Hankook? Serious? If so, do not go too fast or take corners that fast... why the heck did you change them now? You should have simply waited until all 4 new tires arrived.

 

Hit the nail on the head there alex

 

The problem was they had already mounted the fronts and I didn't want to redo them. I guess I should have left both rears the Hankooks. Oh well, I just won't drive it for a few days until the new tire arrives.

 

Good grief. I missed the 3 tires were a different brand than the 4th.

 

For 3 days until the new tire arrives? Yeah, no problem at all, just leave PSM on and don't push it hard. The risk is that the oddball tire will cause the car to perform unpredictably at the limit under different conditions, not that it will suddenly spontaneously combust or anything. Will be fine for normal street driving.

Since it's 3 days, you don't have to worry about wild differences in the way the tires react at different temperatures or anything like that- your car will simply tend towards oversteer in most conditions and you won't be able to take advantage of the increased grip the PSSs offer. Personally, I would not hesitate in the least to drive it like that, though.

 

The Porsche guideline regarding tire size differences between the tires on the same axle still comes into play. 3 days of too much difference in size could have a diff acting up. A big price to pay for mixing tires...

Even the effective difference in tire size between the brands is risking too much in my opinion.

Really I can't believe the OP is even asking about doing this. I can't believe the OP can't snag a rental car just to be safe. I use these times to try other cars. Gives me a day or 3 driving around in a car and a chance to experience the car as I would use it were I to own the car.

 

You want to know if it's Ok to drive an AWD Porsche with one oddball tire on the back ?

 

The OP (me) has another car to use so it's not a problem to let it sit for a few days. My original question was if there is a problem with difference of rolling diameters. The answer is yes, so I am not going to risk driving the car.

 

I think y'all might be overstating the risks of driving a porsche with one oddball tire just a little. It's OK, really!