98 Cobra Speedometer question????

I recently purchased a 98 Cobra. I have tested next to other vehicles, and am noticing that my speedometer is about 8-10 MPH too high when driving. This also seems to be making my odometer move faster(tested that too). Can anyone help me with calibration or do I need to go to a dealer? If so, is it an expensive job?

Sorry, forgot to mention the car has the stock tire size on it, and I do not think it has gears either. Anything else other than gears cause this?