Speed sensor wheel circumference

[Denis99]

there is a discrepancy in the real time speed on my bike.

The Bosch display will read say 12.0 mph, and the Garmin GPS would read nearly 13 mph.

If I understand it, the Bosch speed is related to the circumference of the tyre , whereas ths GPS unit is probably quite accurate.

I have a e fat bike with 26 X 4 inch tyres, and have measured one full revolution of the wheel as used this as the reference number in the display software.

Can anyone confirm any similar issues?

Does the circumference measure , and therefore the actual speed and motor assist cut out depend on the road speed displayed?

 

[Gubbins]

there is a discrepancy in the real time speed on my bike.

The Bosch display will read say 12.0 mph, and the Garmin GPS would read nearly 13 mph.

If I understand it, the Bosch speed is related to the circumference of the tyre , whereas ths GPS unit is probably quite accurate.

I have a e fat bike with 26 X 4 inch tyres, and have measured one full revolution of the wheel as used this as the reference number in the display software.

Can anyone confirm any similar issues?

Does the circumference measure , and therefore the actual speed and motor assist cut out depend on the road speed displayed?

Yes me. Mine is the other way around though with garmin reading about 1mph slower.
I tried altering the wheel setting and then testing against that garmin at cutoff. And although the bosch speed moved up and or down with the cutoff also moving up or down Acording to bosch the actual cutoff remained constant on the garmin at 15.5mph.
I continued minor adjustments until 2 miles travelled on the bosch equalled 2 miles travelled on the garmin and accepted that the bosch speedo shows 1mph over.
Hope this helps.