TDM900 : Calibration of the speedometer with a GPS

Speedometers are often optimistic instruments, everybody knows that.
So here is a calibration of the TDM900 digital speedometer made with a GPS (obviousely on a race track).

I tried to build up the tables as clean as possible, particularly at low & high speed mainly because it's not very easy to keep a constant speed to get a reliable reading on the GPS which is more accurate.
The GPS I have is a Garmin GPSMAP 76CS, but keep in mind that all GPS have the same accuracy once they are hooked on enough satellites.


Speedometer vs GPS (km/h) :

Speedo 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 200
GPS 28.8 38.4 48.2 57.4 65.8 75.5 85.2 93.7 104 113 122 130 139 149 159 167 178 187

GPS vs Speedometer (km/h)

GPS 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190
Speedo 31 42 53 64 74 85 96 107 117 128 140 151 161 171 183 193 204

Graphic : real speed vs odometer speed


Graphic shows that the difference is linear at about 7%.

The speed sensor is located at the output of gearbox, so speed data varies depending on the actual size of the rear tire (wear, inflation pressure & manufacturer). Actually the speedometer display error was 5% with another tyre and another GPS.

The speed display will also be different when using a sprocket with a different number of teeth (15 or 17 teeth vs original 16 teeth).
• 15 teeth sprocket : actual speed is 6.25% less than the speed displayed on the meter.
• 17 teeth sprocket : actual speed is 6.25% higher than the speed displayed on the meter.


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