It probably does affect odometer readings over a rather long period of time, much like over- or undersizing your wheel and tire combination from stock diameter will throw off your speedometer because the whole assembly is going to be spinning faster or slower than it's supposed to.

Also, over time you have to assume that any measuring device is going to get out of tolerance and need a recalibration. We're talking cars that are up to 15 years old...it's certainly a possibility.

My car seems to read a little on the fast side, too, but it's not by much. I notice maybe up to 200rpm difference between what speed the car is indicating and what rpm it should be spinning at that speed. It's most evident to me in 3rd gear around 45mph. I should be right around 3000 rpms, and I'm just ever so slightly past the 3k hash mark on the tach.

Use that nifty little equation to calculate what your speed should be at whatever rpms, and you'll know how fast you should be going at those engine speeds (in a 97-99 2.5, at least).

(RPMS/1000) X 5 X Gear = mph

Example: (3000/1000) X 5 X 3 = 45mph

(4000/1000) X 5 X 5 = 100mph

I'm sure there are ways to do this for the 2.7s with the different ratios and the 3.2s with the 6 speeds, but I don't think the numbers to compute with will turn out nice and round like they happen to for the 2.5 setup. It's pretty accurate.

Yes. The distance from my fuel station and home is 6.6 miles. When I get home in the Boxster on the OEM 16's, it reads 6.8.

All my German cars have read slower than actual.

2005 Boxster 2.5 mph off
2006 Passat 2.0
2010 A6 Just got it, but appears to be in the 2 mph range
2001 SLK 2.0

All figured at 60 mph