I really hope you're talking about political feasibility, but the way you're saying it almost implies you're not. The technology is sitting around to pretty much easily do 60mpg cars - no hybrid required. That would be cars that would look "normal" on todays roads. Look at the VW Lupo...3liters per 100km, basically in the 80mpg range. Then there are zillions of cars that Americans have never heard of that are running around in Europe. Take a look at this link. There are four cars that they tested there that get 45mpg or better at 55mph and only one, the Prius, is available in the US. Hell, the Echo is available in the US and my friend that owns one usually pulls 40mpg real-world fuel economy and I know he doesn't drive slowly. After you look around Europe and see all the great cars they have over there that get fuel mileage that would easily exceed the lame 35mpg standard just put through the Senate you can turn your starry eyes to The Loremo and The VW 1 Liter. The Loremo is slated to be a production car and expected to get 140ish MPG, and the VW 1 Liter which was more of a concept car but also a proof of concept vehicle in the upper 200MPG's. Hat nod to The VentureOne (100mpg) and The Aptera (230mpg) and The Mercedes Boxfish (80mpg, and not at all small or slow)

If Detroit would put as much effort into engineering as it did into marketing and image manufacturing, it might actually build some decent cars. Todays cars are dime a dozen overglorified throw away toys meant to be cheap to manufacture and their aerodynamics are dictated by the marketing department. If the guys on gassavers.org can increase their fuel economy 15% with simple homebrew modifications to existing vehicles like fender skirts, grille blocking and body pans, imagine what an actual effort by engineers with CFD and wind tunnels might acheive. After years of lobbying for and promoting the status quo, Detroit's dug their own grave and it's high time they lay in it.

"In 2002 the National Academy of Sciences did a study which I think concluded that a 25% increase in fuel economy was possible using then current tech."

I'm driving a '95 Prizm. Typically this car gets 25mpg in my mixed high/city use. Simply by driving more conservatively (max 45-50mph, slow starts at lights) I can bump the mileage up by 20%. With aggressive conservation I can get 33mpg.

Thus I question the idea that it would take technology changes to make a great impact on our fuel consumption. The weakest link in automotive efficiency is the nut behind the steering wheel.

Electric utilities have discovered that immediate feedback on consumption very quickly results in voluntary individual conservation. Perhaps what is needed is not technology changes to the cars, but instrumentation changes.

Many BMW's (and other cars as well I imagine) have a button on the console that puts the vehicle into 'sport' mode where the computer adjusts shift points and fuel mixture for maximum performance. Perhaps new vehicles should be required to have a software option that places strong performance limits to assist the driver's natural impulse to drive aggressivly. This could be as simple as a 'conservation' button, much like the 'sport' button on the BMW.

Along with this would be a consumption rate meter in a conspicuous location. Perhaps as a head-up display of some sort, so that drivers are aware of it during acceleration, when their attention is typically on the traffic around them. This would help them to understand which driving behaviors consume the most fuel. The indicator would provide both a quantitative measure (cc/s) and qualitative measure (e.g., green or red light of varying intensity).

These ideas do not require any changes to the technology in the vehicles and can provide a 25 to 35 percent increase in fuel economy, driver willing. Once a driver has learned the driving habits of conservation the gains extend to other vehicles he drives as well.

So, future legislation might pick the low-hanging fruit by requiring car makers to include a performance-limiting dash switch that limits the fuel consumption of the vehicle at the users option and a short-period moving average fuel consumption rate indicator. This allows car makers to very cheaply encourage conservative behavior without requiring them to invent new technology or to significantly reduce the performance characteristics of their offerings.

As an extension auto makers might include a very simple short range wireless communication module that would allow vehicles to share their consumption information. Each vehicle would compare it's own consumption rate with that of the vehicles around it and then display to the driver his ranking. This would be designed to encourage competitive conservation. (Some people would cheat by broadcasting false signals of course, but if widely deployed cheaters would have minimal impact)