Rotaries (I know very little about them) seem to be the most efficient design of all ICE... but are damn complicated as I understand it.
My only experience was with the RX7, and a first generation one at that. Efficient is not a word I would use for a 2000lb car that couldn't break 20mpg. That said, that car was a riot on twisty backroads of which we had plenty. And even though I thought I was going to die several times in it, I never did.
I guess being that fun to drive might have been what kept the mpg so low.
Yes, they always were thirsty, and apex seals seem a vulnerability. Bugger-all torque, but great top end. They are physically small for their output too. I remember one example to illustrate: a Datsun 260z which had the stock 2.6 I6 replaced with a turbocharged 13B. It left acres of space in the engine compartment.
Driving it was something else too. It had a very lumpy idle, and nothing below 5000 revs. If you got up it, at about 7-8000 revs things just went nuts, and your head would slam back into the headrest. It still had the stock tacho, with a "change now" light added, so I don't entirely know what revs it would pull - the needle was off the scale and it was howling. Huge fun.
A few of my friends owned them, examples including an R100, an Rx3, Rx7 and Rx8. My neighbour currently has an Rx2, just as a weekend toy. It is a rorty sounding little beast too.
