Do we actually think that is people knew driving faster kills gas mileage, they would drive more slowly? Or that higher gas prices, combined with that knowledge would make the do so?
I'm a proponent of making as more expensive (among other things). But I lived in Germany, where gas was expensive. (A quick google says average price of gas is 1.94EUR/Liter, or about $7.64 US/gal.) I can assure you that it didn't cause slow driving there. While the US image of German roads without speed limits is somewhat untrue, there are plenty of fast cars. On sections of the autobahn with no speed limit, there is a speed advisory of about 80mph (130kmph). Some cars go slower, but many got at least that fast, or faster.
So I'm not sure that more-expensive gas would result in a significant number of cars reducing speed by a significant amount.