[valkyry1]

No, the theoretical limit is the energy in the fuel, which is about 275 mpg. The problem is that most of the energy is wasted producting heat

Thats why he referred to the theoretical max efficiency of the ic gas engine. The fuel has to combust so how will it get around the problem that most of the energy is wasted in heat?

[Yardstick]

Exactly. We studied this in thermodynamics class years ago. I can’t remember what the name of the ideally efficient engine was — Carnot Cycle or something. But even it was something like only 70% efficient. And I think it assumed no heat loss or friction. The only losses were due to entropy (IIRC, and it’s been a while!).