While not instantaneous, even
very finelycalibrated fuel gauge would help. It might at least permit you to calculate your "tank mpg" daily, rather than waiting for fill up time.
Krousdb pioneered this mod with his Del Sol (pre
MPGuino days): he hooked up a digital multimeter (DMM) to his fuel tank's sending unit, measuring resistance I believe. Then he filled the tank a half gallon at a time and charted the results from the DMM as he went.
Assuming you always park in the same place (fuel tank at a consistent attitude), you may be able to get accurate daily/trip MPG readings from this to help you figure out best mods/techniques more quickly.