H20NOO

I just can't believe how fast I'm running up the mileage on my car! I'm a bit worried that the wheel/tire combo I installed may be a small part of the problem. The big part of course is ME!

The 285/30/18 S-02 rear tires rotate at 837 revolutions per mile. The stock size 255/40/17's turn only 795 revolutions per mile, a difference of 5%. I've racked up about 7400 miles since July so that would mean the odometer is reading about 370 miles too high. I'm assuming the odometer is driven through the rear wheels (i.e. tranmission, etc.), not the front as is the case on motorcycles.

Are Porsche 993 speedo/odo's generally accurate with stock tire sizes? Is the speedo/odo driven off the rear wheels? Can it be adjusted to compensate for different tire sizes?

MC

STLPCA

Matt
Where did you get the comparative rpm(mile) figures? Are they for the same brand/model tire? The 255/40-17 # seems way too low to me. While the actual overall diameter/rpm varies from tire to tire, I would be surprised if the variance between those two sizes is more than 1-1.5%.

The speedo can be calibrated by any decent speedo shop. Should run about $150 -$200.

Viken

Specification / Sidewall / Radius / Diameter / Circumference / Revolutions / Difference

255/40-17 / 4.0 in / 12.5 in / 25.0 in / 78.6 in / 806/mile / 0.0%
285/30-18 / 3.4 in / 12.4 in / 24.7 in / 77.7 in / 815/mile / -1.2%

These are mathematical calculations. Tire sizes vary from one manufacturer to another. I think a difference of 1.2% or thereabouts is not significant.

H20NOO

Dan - You are correct! I pulled the mfg. tire specs from the TireRack website. Here are the Bridgestone S-02 specs.

I inadvertantly used the rev. per mile figure from the 255/40-18 tire. The 255/40-17 tire turns 840 revolutions per mile, a difference of less than 1% (.00358 to be exact).

So it looks like the differences are minimal:

S-02

255/40-17 840 rev. per mile
265/35-18 835 rev. per mile
285/30-18 837 rev. per mile

S-03

255/40-17 830 rev. per mile
265/35-18 820 rev. per mile
285/30-18 840 rev. per mile

P-Zero

255/40-17 827 rev. per mile
265/35-18 828 rev. per mile
285/30-18 836 rev. per mile

It just seems like the odometer racks up mileage about 20% faster than my other cars. I know I'm not driving it 20% more. On the contrary, it's driven a couple days a week on my standard commute and occasional weekend runs "up the hill".

I suppose it's like the saying "time flys when you're having fun". The miles fly by when you drive a P-car!

MC

Time's fun when you're having flies!