[christofoo]

I've thought this many a time.

It doesn't need to be a different form-factor from the existing FE instruments, MPGuino, SGII, UG... it just needs another "gauge" on the UI that displays "Engine Efficiency %".

The only question is how will it map BSFC? Will the firmware have a BSFC map stored in memory for every engine? That's a logistical problem, but it could be done.

Otherwise it needs some additional sensors, i.e. windspeed and incline (or altitude), to be able to calculate on the fly. Even then it wouldn't know (and shouldn't assume) Cd and Crr, so it would need to express the result on the UI some other way than "Engine Efficiency %", for example "MPG-trip-predict" - the MPG you'd expect to get over a round trip at your current efficiency. If this instrument were implemented in a smart-phone, it could use the GPS for altitude (although of course GPS has more error in altitude than it does in latitude/longitude, so the readback would be slow or noisy). Or if it were implemented as a regular FE instrument it could have an accelerometer it could use to get the slope directly (although you'd have to have a method for aligning it, and it would have to be fixed to the panel reasonably well. I also don't think it would work that well considering the vehicle's suspension allows the car to tip, which would create false hill readings during acceleration and deceleration). Arguably it could neglect windspeed. Let the user figure out that wind is hurting/helping their mileage. Actually, I lied about neglecting Cd and Crr. It needs to have some idea how much drag the vehicle has, and you'd need to program the weight of the vehicle along with the other calibrations, so it will know how many miles you can coast at a given speed. But drag is something it could learn on its own, which is best since it will end up factoring engine breaking, which is helpful especially since it may depend on the driver.

I've always thought FE instrumentation could be designed to be far more instructive to new users. MPG is not the most helpful number. Trip average MPG is only a little better.