It's a horrible idea. The market has already been doing an impressive job of improving fuel economy on its own as gas prices climb. That's what the free market does best- it's the most efficient way we know to allocate resources, when resources are constrained.
But in order for it to work we have to stop interfering with oil & gas, too. We have plenty of the stuff right here n the US. But we do everything we can to frustrate drillers- we won't open up offshore areas. We won't open up anwar. We harass people in the bakken with frivilous lawsuits, and refuse to build the pipelines we need to get oil from canada and the bakken to the market.
So we artificially constrain supply then try to compensate by raising the fleet AVERAGE to above the current best fuel economy vehicles on the market. What if I don't want the $35,000 shoebox on wheels this is going to mandate?
edit: here's a summary of what we can do if we unleash oil exploration in the US:
Oil supply is limited only due to bad policy. We don't need expensive, arbitrary mileage standards.