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Discussion Starter · #1 ·
One of the rear tires on our STi got punctured badly enough to require replacement. I won't bore you with the details but for various reasons we had to have the tire replaced that day, i.e., we couldn't wait the several days it would take a shop to order us 2 new Potenza RE070s to match the fronts. We ended up with Sumitomo HTR+. Not fabulous tires or particularly pleasing to look at but relatively inexpensive and again, we needed something quickly. According to the sidewall graffiti they're the same size as the stock tires, but once they were mounted, we noticed some severe "floating" in the front end, particularly noticeable at around 70 - 80mph.

We measured the Sumitomos today and they're about a quarter inch larger in diameter than the Potenzas, but that doesn't seem like enough disparity to have that much effect. We also double-checked each tire's pressure and found them to be ok. We decided to switch the new tires to the front to see if that would make any difference, and it did - the extra swaying has been eliminated.

The handling is basically back to normal now so there's no real issue here, but I'm just wondering if anyone else has experienced this or if anyone can explain what happened - why would it have had that effect when the new tires of the same size were mounted? And why would it make a difference whether they were in the rear or the front?
 

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:eek: Did you shave them to match at least? I wold have bought 4 new tires just to be safe. The diff doesn't like having different tires on that don't match. Also I wouldn't run them to long like that just to be safe. Perhaps the softer sidewall's of the crappy tires caused the issue or something. RE70's have a very stiff sidewall unlike other street tires. Don't burn your diff up it'll cost a lot compared to new tires. Perhaps someone with more experience will chime in.
 

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Discussion Starter · #3 ·
eep, now I'm scared. :confused:


No, we didn't shave them, and just by looking at the new tires you can tell that the sidewall is softer than the Potenzas, since they have a slight bulge like "traditional" tires rather than a nearly vertical aspect like the Potenzas. My husband is extremely mechanically inclined but when it comes to tires and how they interact with the AWD and differential stuff .....not so much. We're pretty much noobs in that department (in case you couldn't tell). This is our first AWD car and basically the only "given" I was aware of is that you don't let it get towed away on 2 wheels - it requires a flatbed. Didn't know the given about changing all 4 tires instead of just 2, heh. I never would've guessed that sidewall constitution would have that much impact, although I guess it makes sense when turning....

Can someone explain this interrelationship in a way that my pee brain can understand it? And how much damage to the diff may I have caused by driving on different tires? Note - I really only drive the car to work and back, but work is about 40 miles from home so I drive the interstate nearly every day. I don't do any autox or track or anything like that.
 

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basically what you have is a mechanical link trying to spin all four tires at the same speed, but if some tires are a different size then they require a different number of revolutions per mile which translates into different axle speeds. The center differential would then be tasked with trying to equalize the difference in axle speed, thus causing excessive wear to the differential. How quick will it fry the diff? I couldnt tell you, but I know they arent cheap...
 

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Discussion Starter · #6 ·
thanks!

We took the advice and yesterday we had 2 more Sumitomos mounted so all 4 tires match. Better safe than sorry.

Thank you to everyone for your advice, and for your patience with our noobness. :redface:
 
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