If the output from your tach is a sinewave then the output from this circuit will decrease in duty cycle with increasing input frequency.
You can increase R1 to 100k and R2 to 1M but then the load created by the EC might be a problem.
Another option would be to omit D1, reduce R2 to 100R, keep R1 at 1k, and change Q1 to 2N7000 which is a MOSFET and has a very high gate resistance >100M. This will only work if the output from your tach is at least 3V peak, I assumed you're talking about RMS when you said 3V to 5V.