we do have 50 mpg cars on the road today - a lot of them hybrids. People just don't seem to like them. I average 17-20 mpg with the GT, and about the same with the Jeep. That isn't bad mileage (Jag gets between 4-8) considering. Most people I know drive further and burn more gas than I.
adn if you have a mix of different-fuel cars, then you'd have to have a mix of different-fuel gas stations to support them. Deisel cars didn't take over the market (like we were told) in part because the fuel delivery infrastructure isn't there. 30 years later it still isn't.
And deisel is far more common than exotic plant-based fuel is.
Hybrids DO NOT get the EPA stated mileage and do not do significanlty better than comparable traditional gas engined cars. The VW diesels have gotten 50 MPG for a long time, though.