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      04-26-2012, 08:01 AM   #6
driverman
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Drives: 2008 328i 6MT - SOLD
Join Date: Nov 2008
Location: El Dorado Hills, CA

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I have a similar question.

The diameter of the stock 225/45/17 front tires is 25" (833 revs/mile)
The diameter of the stock 255/40/17 rear tires is 25".

Since Michelin doesn't make the 255/40/17 for the PSS, there are 2 options:

The diameter of the 245/40/17 rear tires is 24.7". (841 revs/mile)
The diameter of the 245/45/17 rear tires is 25.7". (810 revs/mile)

What difference would the 10mm narrower and 4mm shorter (98 mm vs 102 mm) than stock 245/40/17's make in handling? It seems like narrower would reduce understeer and shorter would increase understeer. Would there be any noticeable difference in handling?

How about the 245/45/17's? They are 10 mm narrower but 8 mm taller (110 mm vs 102 mm) than stock. Once again, 10 mm narrower would reduce understeer but 8 mm taller would also reduce understeer. Would the net effect be oversteer?

The 245/40/17 seems like the better choice. That's what the e46 sedan with sports package had in back with 225/45/17's in front.

Regarding speedometer error....

I think our speedometers normally read at least 5% too high. If we went with 245/40/17's in the rear, the circumference would be about 1% smaller, which would help reduce the discrepancy.

Last edited by driverman; 04-26-2012 at 10:09 AM.