Long shot but I've seen it work before:

Guy gets pulled over for speeding (in a chevette of all things). Being a mechanic, the guy goes to the chevy dealer and gets the appropriate speedo gear to make the speedo read off by 10-15 MPH, then takes the car to a speedo shop to have it checked. They verify the speedo is off by 10-15 MPH, and the judge throws it out as the equipment was not functioning properely and he has a receipt that it was fixed and working right now.

Not sure how that would work with the newer cars and electronic speedos, but the old gear driven ones were easily swapped in a matter of minutes.

Good luck.