Speedo error help

[Ashmore]

I just purchased my first Z3, a 2001 2.5 with only 5600 miles.
I am pretty sure I ahve a significant error on my speedo and odo...when I am doing 75 mph as verified by a couple sources, my speedo says 82 mph.
Is this common, and is there a reasonable correction.
I am running the original tires and wheels.

 

[IslandDawgZ4]

All speedos have some degree of error . I recall reading some time ago that BMWs, in general, are a little higher than most. But, 10% seems a little high. I've checked mine, a Z4 3.0 w/sports pkg, 18", against the local PD radar (we have a permanent radar sign) and it consistantly reads 2 mph fast, i.e., speedo says 60, radar says 58 mph. Against my other cars, current and previous ones, the differences were about the same. The CLS is 2 mph slow (verified by the on-board digital speedo); my RX-8 was 2 mph slow (also digital); and my Miata was 4 mph slow, all against the same radar. Only wifey's previous G500 was different...it was 2mph fast! As BMW had/has different wheel/tire options for the Z cars (without differing rear end ratios), the error could be due to that, I suppose, but that's just a guess. The dealer will probably tell you "they all do that"! The only other solution would be the change the speedo gear head, a costly solution I think versus just being aware of the error when you drive. Can you find a measured mile on a highway and have someone time your speed with a stopwatch? Set the cruise at 60 and see how long it takes. Just to make sure...
Another possibility is that you purchased a damaged car. You didn't say what year Z3 you have, but 5k miles on a minimum 2 year old car is low. Did you get a CarFax report? It might be worth it to do. You can get it online at CarFax.com for a small fee. It should tell you about the car's history.