Just had a thought - probably not original:
Instead of buying a vacuum gauge, another option is to wire a cheap multimeter to the
MAP (manifold absolute pressure) sensor (if your car has one).
It wouldn't be too hard to drive a little bit and see what voltage corresponds to the lightest load, then use it as a relative indicator of fuel consumption. Very useful if using the DWL (driving with load) technique.
Maybe make a simple LED array that does the same thing, so you don't have to read numbers.