tomctomc does his exactly as I do--use a gps and figure the percentage slow/fast from the speedometer reading and then get drive/driven speedo gears to correct. I also have a gps unit mounted in the right wall of my glove box so one is looking right at it with the glove box door open. The gps is not affected by gearing or tire size and it also gives the speed limit which is a ticket avoider on unfamiliar roads.
