Wanting high accuracy and precision? String won't cut it 😀. Doesn't account for weight on the tire which reduces the radius, hence the diameter. Best to do a "roll-out test", where you weight the tire with the expected mass, then measure the distance of one or more revolutions.
But you still haven't accounted for varying weights or tire pressures! Those will also impact your accuracy and precision to some (probably trivial) degree.
Specific discussions of precision were addressed above. AT BEST, with a single sensor (magnet), your precision is +/- 1/2 the diameter and is theoretically CUMULATIVE (+ one time, - the next and random, so it can't be calculated)!
If you wish to increase your precision you will need to add more sensors. Two sensors, you're at 1/4 the diameter. What the heck! Put a sensor on each spoke, you're now at 1/64th the diameter for a 36-spoke wheel😱. (But at what increase in rotating mass?)
Yes, I know, reductio ad absurdum. Two digits is more than enough for me. But that's just me. YMMV. Just ride. /<snark>
Cheers!
Last edited by GAJett; 07-12-17 at 07:23 PM.
Reason: corrections