I think the Mooney has a great NMPG figure. Maybe even better than the TB20?
So Vari-Eze and CT are reasonable contenders.
Here's another I've not flown for a few years - Grob 109b: 15 litres/hr at 115kn or thereabouts if memory services, and on a nice day and not in a hurry, turn the engine off and try and thermal your way there.
I'm impressed with the aircraft's ability to do 207 mph and also get 60 miles per US gallon at 160 mph. That's equivalent to (throttled back to) 10 liters/hr and meanwhile doing 140 kts if I'm not mistaken. I don't think he's exaggerating but I don't quite know he does it.
It's simple. The Cessna 182 SMA (either first generation secondhand, or the new JT-A if you have deeper pockets).
Nothing else carries the same load, the same distance at the same speed - for perhaps £20/hour (130nm). And you can even start and finish your flight on a short, rough, farmstrip.
To answer this question, a good place to start looking would be the CAFE foundation website.
CAFE being an acronym for "Comparative Aircraft Flight Efficiency". Perhaps unsurprisingly, Pipistrel (with its gliding pedigree) have cleaned up at the CAFE foundation's competitions in previous years.
Their Virus SW (bloody awful name, but an aircraft I would love to fly) will do 147kts at 75% cruise which apparently approximates to 18 litres/hour or low 40's mpg. The 80hp variant manages slightly better with low 50's mpg.
Another interesting development in the Austro AE80R rotary engine. Rotary engines are typically pretty lousy at burning fuel efficiently (just ask any Mazda RX7/8 owners). The new engine will have decent engine management, but it's unlikely Austro will do any better than Mazda in squeezing out more efficiency. The interesting thing is the weight of the AE80R will be just under half that of an equivalent 912UL engine (27kg vs 60kg) . Although clearly air frame dependent It will be interesting to see how much of the low efficiency of the rotary will be mitigated by it's reduced weight.
The new engine will have decent engine management, but it's unlikely Austro will do any better than Mazda in squeezing out more efficiency.
Well, not quite as there is an important goal for automotive engines that does not apply to aero engines: clean exhaust. Wankel engines are hard to build in a way that they meet modern requirements and this is also why Mazda finally canned it -- no chance to get it to meet Euro 5/6. The SMA diesel doesn't use common rail injection because it is not required to meet any environmental requirements.
If you look at specific fuel consumption of automotive diesel engines, you will notice that it has gotten slightly worse in the last couple of years -- due to changes targeting clean exhaust and compromising the efficiency goal.
I'll bow to anyone who has greater knowledge of rotary engines than I (fairly likely), but I was always under the impression that the failure of rotary engines to meet exhaust emissions was a primarily consequence of their inefficient operation. If they can be made more efficient, they would likely pass.
A side effect of their inefficient operation is chucking out a significant amount of unburnt/partially burnt fuel out of the exhaust (plus oil, which they tend to have a "not quite Yak" like appetite for), and it is these incompletely combusted emissions which fail the tests.