I tried, but not finished yet. I bought two FCH-Mini sensors http://www.tedcomp.se/produkter/flodesmatare/flodesmatare-fch-mini/ with 9000 p/l resolution. My engine fuel flow is between 0.25-0.6 l/m. One problem is with low RPM, when fuel flow is very small and due to pulsation pump. Both fuel lines seem to require good pulsation dampers, which I do not have yet. Without dampers fuel sensors seem to calculate back and forward causing false counting.
You can test you ranges simply:
- just open line between electric pump and engine and let it run fuel e.g. 10 seconds to some cup big enough and measure litres. Be careful, since free flow may be pretty big.
- open line between tank and return pipe and run engine freely for e.g. 30 sec and measure litres.
- repeat above for 3000 RPM.
With Teensy you can then measure both frequencies with FreqMeasureMulti GitHub - PaulStoffregen/FreqMeasureMulti: Measures the elapsed time during each cycle of up to 8 input frequencies.. Anyway you will need some low pass filtering for measurements.