I drove my Z at 60MPH according to the GPS and watched the odometer. It took 52 seconds to go an odometer mile. I'm 15% fast. I'll look into changing the speedo gear next.

Thanks for the help.

I would suggest doing a much longer test. Get on a highway with mile markers and note your odo as you pass one. Continue to drive for at least 10 miles (more is better). At the 10 miles mark, record the odo reading and determine the error. GPS will get you close, but one mile is not enough for an accurate reading.