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Old 06-05-2019, 09:53 AM   #1
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Rear Tire size question

Hello folks!

It’s time to change the rear tires ( front ones are still good for a year I suppose) since they tend to become dangerous now. Here is my question:
I have a friend of mine who has a spare of used tires 245/40/17 and I was wondering maybe I can switch to them for like a year until all 4 tires need replacing. Since I want to buy a 4 set of Michelin Pilot sport 2 maybe I can use the used tires for a bit ?

The problem is that the Boxster is supposed to wear 255/40/17 and not 245/40/17. You think I’ll have an issue if I use the later one?


Thanks
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Old 06-05-2019, 10:54 AM   #2
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More than the size difference I would not use your friend's rear tires with your fronts unless they were all the same brand and model. Also your friend's tires should have a manufacturing date that is not more than 6 years old, the tires should not have any dry rot and should have a decent amount of tread left. Hopefully someone else will advise you on the size difference.
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Old 06-05-2019, 11:04 AM   #3
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If you have PSM there is a possibility that the different wheel diameter will confuse it and cause interesting things to happen. As a rule, the outside diameter of the wheel should not change, unless you do so for all four wheels. PSM uses rotation sensors in the wheels to determine whether they are slipping.

A 245/40/17 has a smaller outer diameter than a 255/40/17.
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Old 06-05-2019, 01:48 PM   #4
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Quote:
Originally Posted by Lacostas View Post
Hello folks!

It’s time to change the rear tires ( front ones are still good for a year I suppose) since they tend to become dangerous now. Here is my question:
I have a friend of mine who has a spare of used tires 245/40/17 and I was wondering maybe I can switch to them for like a year until all 4 tires need replacing. Since I want to buy a 4 set of Michelin Pilot sport 2 maybe I can use the used tires for a bit ?

The problem is that the Boxster is supposed to wear 255/40/17 and not 245/40/17. You think I’ll have an issue if I use the later one?


Thanks
Need more info:
Which tires exactly are on the car now vs the 245s? Are they compatible in grip and handling? How old are the spare tires?

Which year and model of car exactly? A 986 can wear pretty much any tire but a 987 gets finicky if you change sizes.

Does it matter to you if the car has significantly different handling with unmatched tires?
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Old 06-05-2019, 02:11 PM   #5
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You are talking about tire width here. 245 Millimeters. A difference of 10 Millimeters in width from the 255's. I don't see a problem.
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Old 06-05-2019, 02:29 PM   #6
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Some 245's are virtually the same actual width as 255's
Also check the actual diameter
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Old 06-05-2019, 02:48 PM   #7
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Quote:
Originally Posted by Anker View Post
If you have PSM there is a possibility that the different wheel diameter will confuse it and cause interesting things to happen. As a rule, the outside diameter of the wheel should not change, unless you do so for all four wheels. PSM uses rotation sensors in the wheels to determine whether they are slipping.

A 245/40/17 has a smaller outer diameter than a 255/40/17.

Wheel diameter is irrelevant, tire diameter is what matters.
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Old 06-05-2019, 03:06 PM   #8
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Quote:
Originally Posted by Lew View Post
You are talking about tire width here. 245 Millimeters. A difference of 10 Millimeters in width from the 255's. I don't see a problem.
you're also talking about sidewall section though, not just width.

I assume you understand this, and were making a comment that the difference is negligible. ?

For any who may NOT understand it, the 2nd set of numbers (in this case, /40/) is what's called the "Aspect Ratio". A ratio is such because it's "relative" to another number.

So, for instance, a 225/40/ has the same aspect ratio as a 255/40, but the sidewall height (and therefore the circumference of the tire) is dramatically different.

However: in the case of a 245/40 vs a 255/40; in my experience, you'll see a larger difference in circumference between brands of tires than you will in that size difference.

here:

245/40-17 Sidewall=3.9", Radius = 12.4", Diameter =24.7", Circumference = 77.6", Revolutions/mile = 816
255/40-17 Sidewall=4.0", Radius = 12.5", Diameter =25.0", Circumference = 78.6", Revolutions/mile = 806
a difference of 1.3%
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Old 06-10-2019, 09:43 AM   #9
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Thanks for the input guys. Well it seems that I won’t use my friends tires. The only tires that I can find now (ready to buy and install without ordering etc) are the PS2. But for the front I’ll keep my Bridgestone Potenza N4.

Any implications on this? I know it’s supposed not to mix tires but I always wanted PS2. Front Bridgestone tires are still good for a year ..
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