What throws off speedos?
Is it the 520 conversion or non-OEM gearing?
If you go to a 520 you will need a speedohealer or speedodrd. and when doing the calculation do so at a high rate of speed to have accuracy where it counts. Just my $.02
wrong chain size has nothing to do with your speedo reading, its only when you change the amount of teeth on either the front or rear sprockets
If he is only changing the chain size, but not the actual gearing, then his speedo will not be affected at all.
Fixed
I think the different chain pitch would affect it ever so slightly, but Im not sure if actually would at all, and if it did would it even be measurable. I have seen some gearing change calculators online that ask for chain pitch as well as tire size changes
Even a stock bike has a certain amount of inaccuracy, I can even be different % from bike to bike of the same model and year. Just like HP output can vary due to manufacturing tolerance range ..
Point is you can get it close, but you will never get it exact because as your tire wears it changes....So youve got to accept the fact there will always be a certain amount of inaccuracy