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Old 07-10-2008, 12:03 PM
anE934fun anE934fun is offline
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Location: S.F. Bay Area
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Mein Auto: 2010 335d
Originally Posted by Orient330iNYC View Post
easy... tire size is: tirewidth/sidewall height (expressed as a % of tire width) / rim size.

stock tire size is 225/40/18


3.54*2= 7.08" +18"= 25.08" diameter

spare tire size: 135/80/17


4.25*2=8.5"+17"=25.5" diameter

25.4 is the mm -> inch conversion.
Interesting calculated results. For comparison, I have included data for the published spec on the Bridgestone Potenza RE050A RFTs:

Published Spec for Bridgestone Potenza RE050A RFTs in 225/40-18:
Diameter.........25.2 inches

Based on your calculated result of 25.08 inches, there is apparently some minor variance between expected (calculated) and actual (published specification) diameter. Which is why my interest in your actual results. If you have ordered the BMW spare, when it arrives, could you do the following:

1. Mount the spare on your car at specified inflation pressure (60 psi?).
2. Roll the car forward for 1 complete revolution of the tire.
3. Measure the distance covered by the 1 complete revolution of the tire as loaded by the weight of the car.

The distance in item 3. above would give us the loaded circumference of the spare tire. It is then a matter to divide that result into a mile to get the number of rotations value. I acknowledge that the value won't be at speed, but I am wondering how close the rotations result will be to published specifications. If we wanted to get more precise, we could repeat items 1. - 3. for the Bridgestones and make an adjustment based on circumference for the Bridgestones.

I am personally really curious about the result of the measurement for the BMW spare tire. If the measurement is in the ball park, you may have found a viable option.
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