How to fix tachometer errors on Arduino?

Question

Arduino

The first time I use an arduino, I decided to make a tachometer. The principle is this: there is an incremental counter on the interrupt, once every 100 milliseconds, the revolutions are counted according to the formula
rpm = (RPMImpCount * 60UL * 1000UL) / delta;
Once every 250 milliseconds, the last calculated RPM value is displayed.
The problem is that, for example, at a speed value of 6000, the displayed value floats within 4800-7200 (all values, I repeat, are approximate).
How would you minimize the error? We need, on the one hand, the responsiveness of the display, and on the other hand, the accuracy of the count.

Answer 1

used so far pulseIn().