How about something that has a resistive element like a MAF and a temperature probe in the gas line. Some sort of controller could be used to crunch the data and produce a variable duty cycle signal to feed to the MPGuino.
How about two MPGuinos, one for gasoline and one for LPG. I'm assuming the LPG system uses a solenoid valve to control the flow of LPG so you should be able to connect an MPGuino to that. It may take some head scratching to sort out how to read a total fuel efficiency but each MPGuino should give good readings for the fuel system it monitors.
When you switch fuels you can switch which MPGuino receives the speed pulses so each can give total distance and consumption for each fuel separately.
If your civic has an injector (for gasoline) at each cylinder then an MPGuino should work on that system. If your civic has a throttle body injection system for gasoline there will be difficulties getting consistent readings.
Sure hope someone could help me. 
