CruzPro offers a digital fuel gauge that theoretically could get you close. There are a few significant drawbacks. First is the calibration issue. They recommend starting with a completely empty tank, fill it 1/16th at a time and make a calibration entry... No way I could do that. Mathematical calibration (instead) is proving to be only semi-useful
Second is that even though it will do calculations on the fly... fuel will "slosh" toward the back of the tanks (at least in our case) while underway... which means the data presented is incorrect because the tanks sensor float is higher (in our case). OTOH, resting data seems to be more stable so calculations noted after a trip (or leg) under known conditions could be useful.
Third is that the gauges are at the mercy of the sensor float. I'm still working that issue with one of ours, even having put a new sensor into that tank. I haven't been able to tell if it's a guage flaw (doubt it), a sensor flaw (?), or maybe a connection issue (leaning toward that, but haven't checked every end yet). This one could be a minor detail in a decent system.
The advantage is that the gauges are semi-inexpensive, compared to a full-up fuel flow measuring system. I've read that mostly once you know you'r fuel burn at given conditions, a full-up system becomes less important...
Chesapeake Bay, USA