I may have been a bit conservative with "well over 80%", but also, I didn't include what is burned in the CAT because that normally goes to waste.
I know he may have meant thermal efficiency, but since he quoted 2 very different numbers in the same paragraph, one of them is gotta be very wrong.
In today's modern vehicles, we are lucky if the 25% of the fuel burns complete...incomplete combustion produces by-products (ie polution). Smokey took an impellor (similar to a turbo) and placed it iinbetween the manifold & carb., heating the fuel to about 400 degrees and by swirling the air/fuel at high speed together, vaporized (almost) the fuel. As the mix entered the combustion chamber, 90%+ of the fuel burned= almost no polution and lots of HP.
However smart and resourceful he was, the laws of physics apply to everyone with no exception. And because of all the trickery he had done on NASCAR cars, I don't find it unbelievable that there were some cheats behind those mpg numbers.
Based on a quick look, it appears he's trying to scavange some of the waste heat, and that's not what I think would be an adiabatic engine. Adiabatic engines are simply engines with little or no additional cooling, but because of the high temperatures, they present some serious engineering challenges. In addition, they're also very bad about NOx emission.