I called Mazda USA and they said there isn't any way to recalibrate a speedometer on an RX8; if a dealer finds it's off, they have to order a new computer for the car! This just doesn't make any sense to me. Does anybody know of a reputable performance shop in the southern California area (Inland Empire preferably) that can recalibrate the speedo on an RX8? Mazda dealers are of course repeating the company line of "can't be done".

I'm driving with a GPS in the car, which I have been using in several different vehicles. According to spec it's off by 2 cm per second, which translates to about 0.1 km/hour.

The diff between measured speed on my RX-8 is very little (never been in a car with so little difference). It's typical off by -2 or -3 km/hour at 90 km/hour (roughly 60 mph). For me that's a good margin.

Remember that tire diameter has influence on the measured speed, which will give some diff between brand new and weared out tires or your summer/winter tires.

It's typical off by -2 or -3 km/hour at 90 km/hour (roughly 60 mph). For me that's a good margin.

I tested my car using a GPS unit and found that the speedometer was consistently 3 km slower than the actual speed of the car at all speeds from 40km an hour up to 160km per hour (I think that roughly 25 mph to 100 mph).

It's useful to know that sort of thing in Australia because in parts of our country you can get a speeding ticket for exceeding the speed limit by 3km an hour (that's less than 2 mph).