So for legal purposes, I see we are talking about a track day here?
They'll both be wrong no doubt, but one more right than the other... I don't know. You need a GPS and you'll figure it out very quickly- once it gets wound up a bit, my R90 speedo runs on about 10% optimism.
After rebuilding my G/S speedo, I tinkered with the spring preload and got it pretty well lined up with the GPS readings between 60-100. Between my old car and old bikes, I really appreciate a GPS for this reason- I like to be able to read a very steady and very sensitive digital speed and not try to decipher and translate a dancing optimistic needle.
You could probably also figure it out with tyre circumference and final drive ratios and and your tacho reading. Don't ask me how though.
The speedo works off magnetic resistance. The odometer is geared (and accurite). Your tacho works off an electronic signal from the ICU or something.
At 7200 rpm in top gear, its all relative anyway, the faster and further you went, the older we all were when you got back, you young sprite.
