I made a video on how I modified my optical tachometer to take a direct input from my hall sensor(or optical sensor). This came in handy for me because of the speed at which my motor ran and the fact that it is a single ring magnet rotor. It made it impractical to use the optical tach. If you have a multiple magnet rotor then you would have to add a divide by circuit with something like a 74ls93 to get an accurate rpm reading or just do the math. In other words if you have an 8 mag rotor you would get 8 pulses per revolution to your tach and it would appear to be going 8 times faster than it really is with this mod.
This hack is for my particular tach based on the AT89C51 micro-controller. Yours may not be the same but will operate on a similar principle. Since the video was made I added a jack so I can plug and unplug to and from the circuit. It is a stereo audio jack and power could be supplied to the tach as well because there is a third unused wire but I never did that. Since then I've built an arduino based motor controller with a tach function built in so I no longer need the tach. It was very helpful prior to that.
Note that it still will operate as an optical tach. When directly wired to the circuit it is very accurate when compared to the o-scope reading.