"PeterC" wrote in message
...

If you can find a measured mile[1] that far enough to eliminate most
errors.
Bear in mind that 1 pass of the magnet adds 2m, even if it moves only 1cm
past the sensor.

[1] I've never managed to find a measured mile but did see in Leeds a sign
"End of Measured Mile". A chap in our group wondered aloud where the start
was - hilarious at first but sensible in that we hadn't seen it.

There are measured markers on motorways, not much use for a bike though.
you could calibrate your car using them and then mark out a distance using
the car.

I just went for the easy option and got a bike computer that uses GPS and
doesn't care what size the wheels are.
Its also more useful as you can log tracks and put GPS coordinates into your
photos, etc.