" I also do a bit of fitering on the value to make it readable (the actual period fluctuates quite a bit on real engines)."

Very true. In fact I do it in a simple way as below:

1. Calculate the current rpm.
2. Check to see if it is within a user defined window of say +/- N rpm. If yes, then don't update the display. If no, then only update the display.

This is quite effective in "presenting" a display without the last digit rolling over silly changes. Though I have done this, I dont know the offical name for this method - must be some kind of software filter ?

And where possible, the window value of N is made part of user configuration data. Guys load this heavily and are happy to show thier boss a very "steady" rpm ;-)

Raghu