re: Would current buyers accept...
"Internal combustion engines are about as efficient as they are going to get, I think, so fuel economy is determined by mass (the main factor in town), wind resistance (a large factor on the highway) and driving habits."
You might be right about that. The top economy cars of the 80's got around 35 mpg.
The new GMC "Volt" only uses a small combustion engine to charge the battery for the electric motor. It gets something like 150 mpg.