A simple digital solution would measure the time between two succeeding edges of input clock and then output ten pulses in the next period of input clock. But if the input clock speeds up, there might not be enough time for all the ten pulses. A remedy could be to output the ten pulses in a period of time which is a bit shorter then the input clock period, to have some headroom for changes of input clock period.
The same happens when the input clock periode becomes wider. Then there are ten pulses at the beginning followed by a pause.
Mahmood, can you experiment with such a tacho your customer has? Try to find out, how the tacho behaves with unsymmetrical pulse trains.
Another methode is to estimate the trend of change of input clock periode by analyzing two succeeding periods of input clock. This methode is plausible if the diesel engine is big and rather slow-acting.
Kai Klaas