GT200: Speedometer vs. GPS Chart
Overview
All modern Vespa and Piaggio speedometers read faster than actual speed. In fact, all powered two-wheel vehicles share the same deliberate error. The exact reasons for the error are shrouded in mystery and surrounded by endless debate. However, every rider should be aware of the amount of error present in their system.

The amount of error in the speedometer is a constant percentage of the current speed. It is not a fixed amount. At slow speeds, the error will only be a couple of miles per hour. At higher speeds, the error could be as high as 9mph.
GT200 Measured Error
This is a chart of GT200 Speedometer readings vs. GPS readings. The measurements were taken with a Garmin eTrex Vista GPS unit. The stock Pirelli tires were inflated about 1psi over factory recommendations. The data was collected by jess, taking pains to make several passes at each speed in order to obtain stable numbers.


Speedometer GPS (Actual)
25 23
30 27.8
35 32.5
40 37
45 41.5
50 46.2
55 51
60 56
Chart
External inline image provided by member with no explanatory text

Conclusion
Based on this data, we can reasonably conclude that the GT200 speedometer is off by about 7% across the board. That is, the speedo reads 7% high. Subtracting 7% of the current speed should yield very close to the actual speed.
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