Sorry to bring this up again but after an hour doing searches and reading old posts I just want to make sure.

I have decided to go with the Michelin PS2's on my 2001 996tt. The OEM size is 225/40-18 front and 295/30-18 rear. I want to go a little larger and was thinking the 235/40-18 front and 315/30-18 rear (from a old thread).

I have and want to keep using the OEM rims. So the question is will this setup work on the rims with no other modifications like spacers or rolling the fenders?

Once again sorry for brining this up, however from what I found it is not clear to me that this is ok with the OEM rims and no other modifications.

 

Should be fine without any mods.

 

Hi. The diameters are very similar, and they do work on 8" and 11" wheels.

 

How badly will that throw the speedometer off? The wheels will be slightly larger, diameter wise, so that would make the car think it was going slower than it actually is right? My car reads a couple of miles an hour too fast anyway, so maybe it would work out just fine.

 

the 315 has a diameter of 25.5". The 295-30 is 25.1"
the 235- has 25.4", and the 225-40 is 25.1"

This will cause the speedo to read 1.6% slower.
Porsche speedos read fast anyway, so this might
make the speedo *more* accurate.
NB: Your acceleration will also be slower by 1.6%
Joe

 

I'm not arguing with you, but could you explain how the diameter of the tire will cause a 1.6% drop in acceleration? I see that it would change the final drive ratio between the engine and the tires on the road, as the wheel would be bigger, but is that directly correlated with acceleration?

 

yes it is. The change in diameter is just like a change in gearing.

 

I agree with that, it is just like a change in gearing. My question is whether or not there is a 1:1 correlation between gearing and acceleration.

 

Yes, assuming you don't break your tires loose, within a given gear,
at a given RPM, exactly linear.

 

no problem, I run those sizes on my stockers....235, 315