Quote "The RX-8 car does quite good mpg so i presume it would run well for gal/hr!" Yes but remember it isn't pulling between 70% (cruising) and 100+%(max) RPM when being driven on the road !! PSRU (propellor speed reduction units) are available for a variety of engines.
http://www.fighterescortwings.com/subsubpage3.html
http://northwest-aero.com/
and many others !!
If you want to compare a 12a,13b,or a 20a rotary for fuel usage/hr/shaft horsepower as opposed to either a piston engine/gas turbine you will be "shocked".
As Genghis the Engineer has pointed out regards prop tip speeds and fuel type's for "regs" 1 other area that I would "suggest" you look at is vibration at high RPM with rotaries.This in itself also causes extra maintenance.
Cheers