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Just a suggestion, perhaps you could connect a digital multimeter to the analog output. These are quite cheap and all you'd need was some voltage divider to calibrate the reading to the analogue output. If you are happy with the readings you could get a LCD panel meter module which has all the electronics on board to convert an analogue input and provisions to scale it as well.
It is my experience that a digital display for a tacho would need a lot of damping to stop the last digit from counting up or down too much if the Rev's fluctuate somewhat - very annoying to watch and hard to read.
This could be done easily with a capacitor across the input, though if you use a panel meter module its update rate is slow enough to compensate by a fair amount already.
I just get in mind that I have made a voeding some time and we used a display to a 7-sigmet display and a little driver where you can input a analoge signaal. You need to use some pot-meters for the final tuch and then it can work to. Meaby I can look some time at home of I have some sheets about it and stuf.