road test it and compare to a GPS unit
i road a 53 bmw for 15 years it was km and the speedo was a good one
my trick is take the first number on the right in the 2 digits for a km reading and times 6
like 20 km is 2x6 12 mph
50 km is 5x6 30 mph and 90 + 9x6 =54
its not exact but its close 11x6=66
DAVE
my rt is ok the 660 and the 500 are mismatched odd ball units i use a GPS