Reading the thread on Bobski's Legend MPG display started by feable brain thinking if it would be possible to make a cheap analog MPG guage. I don't want to reengineer something similar to a SuperMid, nor do I have the skills to do so.
My basic thoughts are to measure the DC voltage (basically the duty cycle) of one of the ECU injector outputs. A 12V DC reading would be 100% DC, 6V would be 50%, etc. This would be a crude way of measuring fuel used. An LM2917 (a frequency to voltage converter) could be used on the Vss signal to produce a voltage proportional to speed. If you fed these two voltages into an Op Amp, would not the voltage output be proportional to MPG? If so, it could be calibrated to be reasonably accurate.
Obviously, I'm not an EE, but I think this could work. I'm on vacation this week, but when I get back to work (assuming I'll have the time), I think I'll try setting something like this up on the bench I have there, using a pulse generator as a simulated ECU injector output and a variable power supply to simulate the LM2917's output.
Any input from someone whose electronics background is stronger then mine would be appreciated.