Not sure what you mean by a "roller", but if you want to measure speed of the bike, I would first suggest hall-effect sensors.

Second, I would forget the frequency and the idea of counting pulses.

Assume you have a wheel that has a diameter of 24 inches, including the tire. Every revolution of the wheel will mean that the bike travelled about 75 inches. If you count the spokes on one side of your wheel, you can get accurate spacing of multiple magnets attached to spokes that are an evenly divisible number of spoke apart. For example, if you have 40 spokes, you could easily space magnets at 5 or , 8, 10 spoke distances, giving you 8, 5, or 4 pulses per revolution, respectively. Then you only need one sensor, and instead of counting pulses, you count the time between pulses. From that, you can calculate the speed of the rim of the wheel every time a pulse arrives, and from that, you calculate the speed in MPH, KmH, or Furlongs per Fortnight. If you only resume looking for another pulse after say, 10ms, you do not even need to debounce (assuming you stick with reed switches).