Correct speedo gear

OK admittedly nit picking here.... what is the tire dia with full body weight on it? (also new vs 1/2 used vs tread bare tire will have an effect all be it small) The gps method will get you the closest and you can decide if you want to be a tad over or a tad under. if the gears were $5.00 each I would say buy a couple and experiment till you are happy but at 40-60 each you want to get it right first time.