Originally Posted by Zenmervolt
You'll have worse mileage than a new engine, but it won't be horrible. One of the biggest reasons that the old V8 engines did poorly was that they were lugging around cars with the aerodynamics of a blimp hangar and 3-speed automatics with a top ratio of 1:1 and no lockup clutch on the torque converter. The actual changes in thermodynamic efficiency for an otto-cycle gasoline engine are relatively small players in the overall increase in highway fuel efficiency.
As far as comparing turbines or steam engines to otto-cycle gasoline engines, that's just ridiculous. It was abundantly clear that phucheneh was not talking about a 1905 Stanley Steamer nor about a turbine car that never saw mass production.
Ironically, I'm of the opinion that gas turbines and steam are both horribly under-explored technology. Chrysler's turbine car was a neat concept that people sucked at driving...and that's all she wrote. Seems silly to me that the efforts stopped there, given what we could do with current technological advances. And all steam needs to work is heat, which combustion engines make more of than they do actual usable power output...
But yeah, aerodynamics and gearing are pretty huge. Heck, I drove a 1971 V8 car with a four speed auto from an 80's car and managed over 20mpg. If it had not had steep rear gears and the aerodynamics of a brick, you'd be talking a minimum of 25mpg on the highway. With a Quadrajet and breaker points.
Another thing to point out- my modern 4cyl car has about 150hp. There are other modern cars with 300+ that beat its highway mileage. Displacement does not have a direct correlation to highway mileage. Just like a 1000w home theater system is not actually
drawing a constant kilowatt of energy, engines are not always utilizing their maximum power output. If two different engines are generating about the same amount of power to maintain a certain speed, they're typically also using a similar amount of fuel.