Do you want this just for a visual readout or do you want to control something with it. If you just want to be able to visually read the RPMs there is an inexpensive optical hand held unit on the market. It requires sticking a small piece of reflective tape (included) on whatever you are clocking, shaft, pulley, etc. I have used one for adjusting generator RPM and it seems to be very accurate in the 3 to 4,000 RPM range I use it for. I think you can find them on eBay for under fifty bucks.