remember when unleaded fuel first cam out? You could stick an unleaded nozzle in a leaded fuel car but you couldn't pump leaded fuel into an unleaded vehicle.
could we do something like that again? suppose that the restrictor plate in the gas tank is sized according to fuel mileage? I.E. as the mileage gets worse the hole gets smaller. And the pumps at the gas station were also sized accordingly, so that all current vehicles and 2007 models that get better than 30mpg pay the current $2.35 per gallon those that got between 15 and 30 paid 2,35 plus say a dollar tax making the fuel $3.35 and those vehicles getting less than 15 mpg paid say $4.35 and the pumps ran slower. so that a guy with an expedition had to take say 30 minutes to pump his 30 gallons of gas. and then each new model year the standards increase so to get the large restrictor in the fuel tank the car had to get 40 mpg and the middle sized one would go to say 25 to 39 mpg and anything less than 24 mpg would get the smallest plate and pay the highest price. this way current owners would not be penalized, and each year as the standards get tougher it the new vehicle buyer who has to make the choice. I think three sizes would be easy enough to do cause minimum confusion and generate big results the first year. If we then took the extra tax revenue and it went solely to energy research (wind, solar geo-thermal, fusion whatever) for other energy needs (home heating, lighting etc.t) then petroleum would only used as a fuel for cars. and as other energy became cheaper perhaps electric alternative fuel cars could make a bigger dent in sales of convential fuel vehicles