I think that we may be off on completely the wrong track in trying to use less petroleum in our vehicles. In effect what saving is doing is playing in to the very hands that have prevented the development of alternate fuel technologies.
I view this approach as a mid-term alternative which buys time until the alternate fuel technologies are available. The only alternative fuel technology that I beleive has a future within the next 10-50 years is bio-fuel. The conversion of agricultural products and by-products into petroleum and petroleum by-products. This is actually a zero effect on the greenhouse gas problem because its only recycling carbon on the surface of the earth as opposed to introducing carbon from underground. Additionally, it will never run out and would be produced in locations around the world that are more polically stable.
I don't beleive any other technology can make a significant impact in my lifetime due to the current technological limitations and in most cases limitations due to the laws of physics (which technology can never overcome).
For example of what I mean by limitations, consider hydropower. While it sounds good to generate electricity by dams, people overlook that most of the prime locations (at least in the USA) have already been damed so there's not much potential for increase here. Also, damning a stream causes great environmental damage as well which is counterproductive to the goal of environmentally friendly energy.
Or consider nuclear. Relatively cheap, proven technology with almost limitless growth potential. But what do you do with the waste?
Other forms of energy have similar problems that I won't go into right now in the interest of space, but suffice it to say we should be more efficient where we can and this is one step in that process.