You query got me thinking. Reading your tach sensor with interrupts should be no problem. However, outputting tach data to your display may foul you up. You probably know this, but I will add it anyway. Don't use delay() when outputting to leds, buzzers, displays. This is often used by "novices" to slow down the output to make it readable. Using delay() would certainly mess up your interrupt read. Use millis() instead. It will allow you to use the clock as a timer for your output without pausing any functions.