Federal law is pretty lax on the accuracy of speedometers:

"Each bus, truck, and truck-tractor must be equipped with a speedometer indicating vehicle speed in miles per hour and/or kilometers per hour. The speedometer must be accurate to within plus or minus 8 km/hr (5 mph) at a speed of 80 km/hr (50 mph). "
[70 FR 48054, Aug. 15, 2005]

That is an error tolerance of plus or minus 10%.

http://www.fmcsa.dot.gov/rules-regulations/administration/fmcsr/fmcsrruletext.aspx?reg=393.82

Seems that this would be a plausible defense if you got a speeding ticket for 5mph or less over the limit.

Todd
'88 Westy