Does changing tire size change your speedo readings? I just put a pair of new 2002 996TT hollow spoke wheels on my 86 930 (feels and looks great). It is hard to tell, but it appears the speedo may be off a bit. If the speedometer measures speed purely by RPMs, then it will be off as the 18" rims and tires are slightly bigger in circumference than the stock 16" fuchs. Here are a few pics:

Close Up -

Side View -

 

Well - I answered my own question. Palo Alto Speedometer has a nifty tire change calculator on their website at:

Speedometer Tire Change Calculator -

996TT Tires read at 59.3 MPH when I am going 60 MPH - pretty close!

 

Stock tire diameter is 24.9 in front and 24.7 in rear. Anything larger that that in the rear will have your speedometer reading lower that your actual speed and vice versa. I used a GPS to measure my speed and it appears my speedometer is accurate with 315/35/17 tires on the rear.

 

ZCAT3,

The wheels look great!

Narinder

 

Thanks Narinder - the difference in ride is surprisingly minor and the steering feels easier and more precise (which is counter intuitive as the tires are much wider).