Wheel magnets are plenty accurate, no need to worry about where they are placed. I always had accurate mileage when I used them. I now use rotation sensors and they are just as accurate, especially when combined with a GPS head unit. It can calculate wheel size based on GPS miles/revolutions, and just take it from there. I switch my sensor and head unit from a 700c road bike to a 26" MTB and I get the same miles on either, and the same MPH, both based on the rotations, not the GPS, without doing anything. And these miles equal the miles my old magnet system would report. I had them running together for a while just to be sure and it never varied by more than .1 over a 50 mile ride.