Agreed on the fuel economy, or lack thereof. If I drove the ones I had like I stole them I got around 20 miles per imperial gallon (about 17 miles per US gallon). In combined city/highway driving I got around 24 mpig, and on long highway trips I got up to 28 mpig.
These numbers are not bad considering the Datsun 280ZX and Toyota Supras of the same era returned about the same performance figures and fuel economy.
I think the rap on the RX7's fuel economy stemmed from two things:
1. Its low displacement, which was (arguably) 1146cc - a displacement that, in piston engines, returned at least 40 mpg back then (albeit, with a zero-to-sixty time of several minutes).
2. Its low horsepower of 101 - (vs. around 145 hp in the 280ZX and Supra).
True, the rotary DOES use more fuel per hp than piston-poppers do - about 10 to 15% more. But that lower hp number in the 12A-powered 1st gen RX7s is deceiving: first, the car was lighter than its competitors, around 2400 lbs. Second, that hp band hangs in there over a much wider power band in the mid to high rpm range vs. the piston engines of the day. Third, the "power stroke" in a rotary engine lasts 50% longer than that of a piston engine - 120 degrees of crank rotation in the rotary vs. 90 degrees in the piston engine.
The end result was a light-weight rotary-powered sports car that produced comparable power-to-weight ratio figures while making it's limited power more useful. Once past the weak-torque-at-under-3000-rpm obstacle (not to mention the 4-barrels kicking it very shortly thereafter) these engines came on strong and stayed there effortlessly.