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Tire Circumference


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6 replies to this topic

#1 bjwirth

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Posted 29 November 2004 - 03:04 PM

Over the weekend, one of my tires blew (sidewall). Now I need a replacement. I've done a search in this forum and it seems everyone is in agreement that the circumference has to be within 1/4" of each other. This makes perfect sense.

What's fuzzy is some people recomemend buying 4 brand new tires, while some say to "shave" the tire to match circumference. Some say that the replacement tire has to be the SAME brand and tire as the others, while others say it doesn't matter.

Ok, so I live in a small town and I called around a few places and nobody shaves tires. One of the guys I spoke with said the circumference is a function of the tread depth (for all 215/60/16's). I didn't think this was true, but who was I to question him. (I thought I recalled a post where 2 different tires had 2 significantly different rolling circumferences) anyway he said he would measure the tread depth and find another tire with a close tread depth.

So my questions are:
1. Do I need 4 matching tires or 4 matching circumferences?
2. If the previous answer is 4 matchng circumferences, can I just measure tread depth, or do I have to get the tape measure out and measure tires?

If these tires were more than halfway spent, I'd probably just buy 4 tires. But the tires are less than a year old (15k miles). Before I knew the board existed, our last car was a 96 imprezza which we had for 8 years. It seemed we were replacing 1 tire every 1-2 years- right from the beginning (due to damage or age). So you can imagine by the 5th year (and beyond), we had 4 different tires, with 4 different tread depths. The car had about 75k miles when we sold it, but never had any transmission problems. Had I known about the tire circumference issue, I would have made more of an effort to at least change the tires in pairs or something. Did I luck out, or was the transmission on its last breath?

thanks for any help/

#2 Setright

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Posted 29 November 2004 - 03:58 PM

The tyres need to be of the same circumference, that's the clincher.

Of course same brand and type will have the same wear characteristics, which will be important as the miles roll by on your odometer.

You can't judge the circumference by measuring the tread depth. No way, no how, no ma'am. Tape measure all the way round.



It could be argued that two different circumferences on the same axle would not be a problem. The difference would be taken up in the open diffs that connect side-to-side.
The main problem, as I see it is if both the front tyres are bigger, because then the partial locking center diff has to take up the difference in revolutions.

I replace all four. But I am a born worrier, and would never get any sleep if my tyres didn't match.

#3 99obw

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Posted 29 November 2004 - 10:09 PM

http://www.tirerack....tchingtires.jsp

#4 bjwirth

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Posted 30 November 2004 - 11:53 AM

Thanks for the replies. From the research I've done and the people I've spoken with, I think I am just going to replace the one tire with a used one. We can easily see the impact of different diameters on our differentials. And I can see how a different tread might affect rotation and traction of the wheel- but only in slippery conditions or turns. I have no way of knowing this, but I gotta think that two "comparable" tires (ie all-season touring tires w/ similar ratings) aren't going to perform that differently.

As for going for a used over a new- a new tire may have a circumference of .25" or less, but a used one will also have the same difference (but opposite magnitude since it's smaller). I'll keep an eye on it and if the used tire wears faster, then I'll just change it with another one or just get 4 new ones later. with my luck, I'll damage another tire in a year anyway.


So here's my next question: does a tire on a rim (more or less "deflated") have a different circumference as a tire off a rim? The tire store I called has 5 used tires in my size. I'd like to bring a string and see which has the closest measurement.

Thanks

#5 Glenner55

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Posted 30 November 2004 - 04:36 PM

I've replaced just one tire in the past with no adverse affects, key is to measure the circumference of the tire accurately with a tape measure or piece of rope, then measure the rope flat on the ground.

I'm running three matching snows and one mismatched snow, of the same
circumference, on my wife's 96 OBW. We bought a used 92 Legacy Turbo that
had mismatched tires on the back axle, the different size made the transmission slip and buck at high speeds as the car always thought the rear of the car was turning slightly.

A similar thread pattern is the way to go so you're getting comparable levels of traction on all four wheels. Have the shop remove one of the tires to measure it or use the damaged one if it's still around.

Glenn

Thanks for the replies. From the research I've done and the people I've spoken with, I think I am just going to replace the one tire with a used one. We can easily see the impact of different diameters on our differentials. And I can see how a different tread might affect rotation and traction of the wheel- but only in slippery conditions or turns. I have no way of knowing this, but I gotta think that two "comparable" tires (ie all-season touring tires w/ similar ratings) aren't going to perform that differently.

As for going for a used over a new- a new tire may have a circumference of .25" or less, but a used one will also have the same difference (but opposite magnitude since it's smaller). I'll keep an eye on it and if the used tire wears faster, then I'll just change it with another one or just get 4 new ones later. with my luck, I'll damage another tire in a year anyway.


So here's my next question: does a tire on a rim (more or less "deflated") have a different circumference as a tire off a rim? The tire store I called has 5 used tires in my size. I'd like to bring a string and see which has the closest measurement.

Thanks



#6 Setright

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Posted 01 December 2004 - 02:48 AM

Well, there is going to be a difference due to the air pressure inside the mounted tyre. However, since I trust you are running the radial ply that difference will be tiny. The tread isn't that flexible.

#7 bjwirth

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Posted 01 December 2004 - 11:34 AM

Well, there is going to be a difference due to the air pressure inside the mounted tyre. However, since I trust you are running the radial ply that difference will be tiny. The tread isn't that flexible.


There is no pressure (or I should say, there is no guage pressure) in the bad tire since there's a gash in the sidewall. I was wondering if stretching the tire around the rim distorted the shape in a way that would change the circumference.




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