One thing you might consider, look at mounting your sensor before the rear tire. You will get better resolution. Or make a reluctor wheel attached to the rear tire. For example, when I was doing data acq on race cars, it was normal to detect drive shaft rpm with a four count reluctor wheel on the driveshaft. I mounted the hall effect sensor on the housing of the rear end and counted ring gear teeth to determine drive shaft rpm. I would get 20 pulses per rev compared to 4.