Re: Speedo's and gas engines...Help !!
First, it matters NOT how far away from the hub the magnet is mounted. Whether it is close to the hub or close to the rim, it still spins at the same rpm. The computer is simply a counter, counting how many times the magnet passes the pickup. It doesn't measure how fast the magnet is moving, but rather, simply counts pulses and then the computer calculates speed etc..
The ONLY way to get an accurate measurment of tire "size" is to have it at anticipated operating pressure, load (panniers, lunch, water bottles, backpack, what-have-you, and on a level concrete surface, while seated on the bike, roll it forward very straight, one full wheel revolution (oh yeah, dab a bit of grease or paint or lipstick or water or whatever onto one single knob of tire tread, and measure very carefully from leading edge to leading edge, or trailing edge to trailing edge, i mean, really be accurate, a 1/16 inch difference times a great many many many many many revolution in a trip can throw things off considerably.
Note: cars aren't "accurate" either. Tire tread thickness and psi affects speed reading. Modern Fords ( which I work on, are biased to read 5mph higher at freeway speeds on purpose...(without increasing odometer reading).