How do I estimate piston aircraft fuel efficiency?
Yes there is a simple rule of thumb for what is known as Specific Fuel Consumption that you can apply to estimate the fuel burn of most piston engines:

An air cooled carbureted engine, leaned, will burn roughly 0.44 to 0.45 lb/hp/hr.
With Fuel Injection, a bit less, maybe 0.41 or 0.42-ish.

Cars with electronic sequential fuel injection are in the high 0.3s.  Diesels are in the low 0.3s

2 strokes are up in the mid 0.5s.

Turbo props are very thirsty and are in the .6 range.

Take a Cessna 150.

75% cruise is 75 hp.
X 0.45 = 33-34 lb/hr.
= 5.6 usg/hr.

Which is about what you will find in the POH, or close enough anyway.

Want to know the fuel burn of an R-985 powered airplane?

450 X .75 = 337 hp
@ .44 lb/hp/hr = 148 lb/hr
= 26 usg/hr, which sounds about right for something like a DeHavilland Beaver.