You guys are way overcomplicating this--Cdowns has it right.
If you are really picky, take the deviation in seconds from 1 minute to travel measured mile (+ 0r - seconds) divide that by 60 for the percentage it deviates--that's the percentage your speedo is off.
IE: 58 sec. = 2 seconds deviation, div. by 60 = 3% fast or 61.8 actual mph. Close enough for government work and you never have to get out of cab or buy a $200+ piece of equipment!
