With the high price of gasoline, lots of people are looking for ways to save money on gas. The simplest method is simply to slow down. The drag on a vehicle goes up with the fourth power of the speed. That implies a very crisp knee in the curve.
Of course, the standard answer would be "lower the speed limit to 55MPH." But that would really be a bummer for people on long trips. We have better technology than in the 70's. Most people cruising down the highway at 75 don't know that they could slow down 10 or 20 MPH and save real money. Let's give them data. Here's my proposal.
Why don't cars come with a meter that shows how much you're spending right now on gas. Turn instantaneous mileage into instantaneous dollars and you'll see real behavior change. That leaves people free to choose and most will choose saving money when there's no compelling reason not to while leaving people the freedom to spend money to get where they need to be.
One step further: create an online game where people can compete for best performance over a given route.
I'm looking for an iPhone app that does this for starters.