I never said that the tires don't make a difference, but it's the front tires and it has nothing to do w/ the transmission or rear tires.

 

All the different tire sizes and wheel sizes used on these cars have virtually identical overall diameters.

 

Oh I get that, Bill. I'm just disappointed that the gauge is off that much with what should be a diameter pretty close to stock.

 

225/40 x18 & 205/50 x17 & 205/55 x16 are the 3 main front tires that came on a 993, all are ~25.1" OD

 

Assuming we are all running approximate factory diameter tires, since JPS and NYC993 have speedometers that show higher than actual while I show lower than actual, the issue is more of quality control at the factory or wear items within the speedometer.

Does this also manifest itself in the odometer? Not that it makes much difference but does my car at 100k miles really have 103k? Does JPS' at 100k miles really have 97k miles?

 

That may be, I rarely look at the speedometer.

 

This could be an easy fix..

http://www.yellr.com/yb_home.htm It's a pulse re-calibrator for speedos.

With 18" rims on my car, the speedo is about 5mph faster @ 75mph actual.

C.

 

That's interesting. Which model would fit a 993? I suppose one would just intercept the signal coming out of the DME?