I don't see any reason to believe why the cars manufacturers make is related to tax. Consumers don't buy higher fuel economy cars in the US compared to other options because they have a choice, and manufactures try to build vehicles that fit demand of the consumers.
I don't care for taxes, but fuel taxes do pay for roads.
If everyone changed to a higher economy car, the impact would probably be that fuel tax revenues would go down. Thus, that would impact money to build, maintain, and repair roads. And you can expect that taxes would be raised. But that has already happened hasn't it with even SUV's and all other vehicles getting more miles per gallon.