I grew up reading everything Smokey Yunick wrote. Popular Science and Hot Rod magazines were a couple of publications I subscribed to, and both carried Smokey's writings on a monthly basis. The reason I bring this up, is this claim and the prolific "run your car on water" scams that are everywhere.
I remember in the early to mid '80s, Smokey unveiled his new design for a high-efficiency adiabatic engine, which he actually built and tested. He used a 3000 lb car (1980 Buick Skylark; not a very aerodynamic car) with a 125 cubic inch displacement, 3 cylinder engine running on 93 octane gasoline, and achieved 0-60 mph acceleration in 9.4 seconds with a very smooth running and very drivable engine. He then ran a 133 mile test loop 10 times, and measured the average fuel efficiency. The test loop was 50% city driving and 50% highway, and took place on public roads, in normal daily traffic. The average fuel consumption was 48.25 mpg. The speed, by the way, was maintained to within 2 mph of the speed limit throughout the test.
Unlike the scam artists, Smokey was very forthcoming about how his design worked and how to implement it. Here are the basics, as I recall them and in my words: Smokey used the heat that the engine produced to superheat the fuel/air intake charge, so as to achieve greater combustion efficiency (in a standard engine, a large percentage of the hydrocarbon atoms do not combust. This is why you won't catch me saying that small amounts of hydrogen added to the charge cannot increase efficiency; only that the "run your car on water" scammers aren't doing it, otherwise they would explain the process). The fuel charge wants to expand back out of the intake manifold when heated, so the system uses a turbocharger not to pressurize the intake charge as is its normal function, but rather as a one-way valve top prevent this reverse expansion.
The next problem is detonation, and Smokey dealt with that by using a space-age ceramic coating (then new, now common) on the piston tops and combustion chambers to insulate and prevent hot spots which would cause preignition.
This system worked so well that the traditional radiator could be eliminated, with only an oil cooler and a heater core being sufficient for cooling.
This technology, applied to a small 2-cylinder engine in a lighter, more aerodynamic car probably could actually achieve 100 mpg while still being able to keep up with traffic.
Not only was Smokey forthcoming with details of how his system, unlike the scammers who hide behind "patent pending" as an excuse for not explaining how their system supposedly works, Smokey actually did patent this technology. It is U.S. Patent #4,592,329, entitled "Apparatus and operating method for an internal combustion engine" dated June 3, 1986 and you can download or read it in its entirety here.
Here are some books Smokey wrote: