Would current buyers accept...
A car with 55 hp? And according to Wikipedia, the Met weighed well under 2000 lbs. Even a Toyota Yaris weighs 2300 lbs, with 106 hp. (40 mpg US - we have one and I can verify)
Maybe people would buy low-powered, light cars with great fuel economy, going without some comfort features like A/C and sound insulation, but apparently the car manufacturers don't think so.
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.
We could go to something like this:
Last year saw a new world economy record set during the event when Team MicroJoule achieved an average fuel consumption of 10,705 mpg - to put that in perspective, that's the distance from London to Melbourne, Australia on less than one gallon of fuel!