Assuming you are using a timer in your tachometer program, use it to calibrate the output. Use a photo tach or accurate tachometer to calibrate. Adjust the timer to count 1000 pulses at 1000 RPM. Display 1000 & restart the timer.

 

If you get 180 pulses per second at idle, it will take 4.5 to 5 seconds to get an accurate number. If you multiply your pulses by 5, you will lose resolution but it will refresh every second or so.