Digital rpm meter without using microcontroller

The main problem is that of accuracy versus gate time. Suppose your engine does 10,000 RPM flat out. If you hold the gate open for 60 Seconds you get a display of 10,000, if only 6 seconds then the count will be 1,000, if you choose .6 seconds then the display will be 100. Now if you throttle back to tick over (1000 RPM) then the display would be (.6 secs gate time) 10. Which does not give you the accuracy to set a proper tick over, as for a start any digital display is +- count, so a display of 10 could be a tick over of 900 - 1100 RPM.
With a gate time of .6 Secs, you need to have a latched display, so while the counter is counting, the display is still showing the last figure which then just flicks over to the new figure. Most counters have a "hold" or "display time" knob, this freezes the display to stop a fast changing frequency to cause all the displays to flicker so fast you can't read them, as your updates are every .6 Secs I don't think you will need this feature. There are chips that contain all these functions and display drivers, so you just add a timing oscillator and a display and you should be away.
Frank

What about counting the period of the pulse ? The period could be converted into frequency.
After that the frequency could be multiplied by 60 to get rpm.

Is it possible to implement those functions in circuit ?

If you know TTL/CMOS logic function you could do it with BCD-counter(e.g. HCT390 one digit) and a one second time base. The first stage must be designed as a by 60 divider follow by as many digits you need to display. At the end of 1 second the value can be latched into a register and then displayed. Thus quite a lot of logic design is needed and the accuracy depend on your time base. If you do it the analog way you would have to charge a capacitor with the RPM-pulses and calculate the function of the output voltage to RPM relation. The display can then be analog. Here are some principles of speed measurement with magnetic/optical sensors: **broken link removed** .