So the only way I can get about 100 MPG is to buy a car that is $80,000 or at least in the upper $40,000?

You should just attach a lawn mower engine to a bike. You could probably get at least 200mpg! I did it for $500

in all likelihood not. a minimum of not a often happening, highway criminal vehicle with room for 4. possibly a gasoline powered golfing cart or something like that ought to get a hundred MPG, yet for the dimensions and weight of a classic economy vehicle, no gasoline engine with sufficient capacity to maintain up with site visitors will furnish that variety of mileage. i don't think of any diesel engine ought to the two, besides the fact that diesel engines do get extra capacity out of a gallon of gasoline than gasoline engines do.