Interesting. My speed is accurate according to my radar detector. At most, it is a 1mph deviation. I am running 18s. However, I heard the detectors get their speed from car wheels sensors, not gps, but my detector is equipped with gps.
My 2002 has stock wheels but is no longer running on the original tires. The tires are of stock size. The speedometer reads between 3-4 mph over the actual speed.
I also wondered if there was something I could do to show actual speed. So I plugged in my OBD reader a scan gauge 2 and found out that the car actually knows what the accurate speed is, so your odometer is correct but some how our speedometers actively convert the speed a percentage up.
I now run my smart phone with the app "torque" and GPS speed shown.
2000 Porsche Boxster
Mazda RX 8 Sport
986
Hybrid Mode
