New tire matched with old tire: issue or no issue? - Alfa Romeo Stelvio Forum
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post #1 of 3 (permalink) Old Today, 06:41 PM Thread Starter
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New tire matched with old tire: issue or no issue?

Stelvio Quadrifoglio with 10,000mi. I got a screw in the center tread of rear tire. Have road hazard insurance. Vehicle in for 10,000mi service at dealer. I ask dealer to sort out the tire. I figure they will just patch it (center tread). Dealer calls back: "good news, hazard insurance giving you a new tire." (These are 285/40R20 Pirelli P Zero). "I say OK, what's the other tire look like?" My understanding (cell phone reception) is other tire has 6/32 wear (6/32 remaining). Half tread left, but guessing another 5000mi +/- on it. This puts the old tire 3/8" smaller in diameter than the new tire, 1.2" smaller in circumference. New tire dia. 29", circ. 91".

So my question is: Is this amount of tire size difference problematic?

From what I've read in various auto blogs, rule of thumb is to replace the tires in pairs, but if less than 2% difference, not a problem. (About 1.3% diff). But it's a little confusion on the topic as some people answer this type question as understanding you have two slightly different tires on the same axle and others weigh in opinions on whether you simple have slightly different tires between the axles. (Which is the case in the Stelvio any way as the front and rear tires are different sizes to begin with). And other blogs, people are weighing in on deviating from manufactures supplied tire size. So sometimes the answers are confusing if you don't understand the assumptions of the commentor.

FYI: dealer quote for new tire mounted, $550.
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post #2 of 3 (permalink) Old Today, 08:03 PM
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I’ve no idea about the Q4 set up but on a quattro if the difference is (I want to say offhand 1/4” difference) you need to change all of them. The reason is the different circumference tricks the awd system as the wheel is spinning at a different rate so it activates the awd.

A certified Audi tech that works for me now has told me that even different brand tires of the same size can cause problems.

Good point on the Stelvio having different sizes front and rear so I really have no idea....

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post #3 of 3 (permalink) Unread Today, 08:08 PM
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Everything you wrote is correct.

I had the same thing happen...…I had a little over half the tread life left when I replaced mine, but haven't had an issue in the last 3000 miles or so

To define "need" though....

The reason you want to replace tires in pairs is due to circumference and traction differences. The new tire, with more tread will have a larger circumference as that wears down as the tire wears. This is not as much of a problem with modern low profile tires as it was with old school huge sidewalls with deep tread pattern tires...think truck tires. With these tires the circumference differences are negligible....the entire tread depth on these tires isn't much more than a quarter inch when new....the difference between new and half worn is pretty small.

The second issue has to do with the new tire having more grip or traction then the old tire which can cause various issues, like pulling to one side and other control issues, and can be very serious. However the Stelvios's AWD system (and most modern AWD systems) is set up to juggle traction automatically as soon as it notices a difference. So while you might cause some additional brake wear as the system manipulates things, you wont damage anything.

Best to replace in pairs in more true for larger tread tires installed on two wheel drive vehicles. The Stelvio or most modern AWD vehicles it really isn't a huge will notice turn in to one direction is little crisper and faster though, you'll need to adjust if you push the vehicle regularly.

What Cornbeef wrote about the AWD activating is true. However does that mean you NEED to replace them all to match? No, not unless you have limited slip and or locking differentials which is what quattro uses in most of their setups. The Stelvio uses a torque vectoring setup that allows for far more adaption from the system. You aren't going to hurt anything, you might wind up wearing your brakes a bit more, however my experience the way the system works it wears down the new tire to match the old one pretty quick...maybe 1000 miles or so.

2018 Stelvio 2.0T AWD

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Last edited by ALFAOFFROAD; Today at 08:15 PM.
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