Vivek, isn't it perfectly acceptable to have an RPM value that is always changing? Your application (without knowing specifically what it is) is most likely constantly changing speed anyway. Perhaps it runs at 1020 RPM +/- 30 RPM. Load changes, power changes, vibration can all effect the speed.

There are other sources of uncertainty too. Your pickup may not be 100% repeatable, i.e. it may trigger at a slightly different shaft angle each time. The pickup points may not be precisely located. Remember, even if you add more points, your signal timing will only be as accurate as the machining tolerances of the interruptors! There could be noise on the signal that could effect the switching point. Also consider the nature of the signal - sometimes one parameter, such as period, can accurately give speed while low-time or high-time is not accurate.

By the way, what is your tach signal for? If it is an operator interface, for example you can consider taking an analog display approach, or you could set the tach refresh rate to a low value for digital numbers.

I used the pulsewidth approach to measure engine speed on a sport bike, from 50 rpm up to 14,000 rpm. My custom dashboard has an LED "analog tach" as well as a digital readout. It responds instantly and gives accurate readings. Counting pulses over a time interval is an inferior approach for this type of application, unless your engine speed is dead constant and you need long-term accuracy.

My $0.02

- Lee