Speedometer error

[simbob]

Has speedometer error been discussed here before? If so, can anybody direct me to the thread?

The Christmas purchase of a GPS and the coincidental holiday placement by the local police of a "your speed is" radar sign station in my neighborhood both agree that my speedometer reads a consistent 5 mph faster than actual speed. Although the neighborhood sign was checked against the GPS and the speedometer at speeds from about 10 mph to 40 mph (speed limit 35), the GPS shows the same 5 mph overstatement at speeds up to 80.

I know that most cars register a speed slightly faster than actual, with the raw mph error usually increasing with speed, and I was used to a 10% error on my BMW motorcycles, but 5 mph seems high to me for a relatively recent vintage car, and the raw numerical difference doesn't seem to vary with speed.

My wheels are stock 17" Boxster design, and the tires are the stock size.

Is 5 mph an unusual speedometer overstatement in the Boxster?

Thanks.

Bob