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