Use a Garmin or a TomTom GPS and record the speedo reading at 10 MPH increments from 20 to 60 MPH. That should give the speedo shop plenty of info to work with. When I switched from 12" wheels to 14" on the GEO Metro I drive about 1200 miles a week, the change was about 10% over the entire speed range and the odometer readings. I'm indicating a pretty consistent 10% slower on the speedo than the GPS. The error increases as the speed goes up.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!