Long term, I see hydrogen and oxygen either in a fuel cell hybrid electric, or internal combustion ceramic engine running at more efficient higher temperatures.
Nearer term, perhaps a reacter could drive combustable liquids or gasses from coal, wood, food compost, manure, or even human excrement (put pollution byproducts to good use).
Short term, I would probably be looking at turbo diesel, and hybrid electric vehicles. Maybe biodiesel using peanut oil (indirect solar and a regenerative resource). If need be, As a less preferred choice, I could see using alternate fuels such as alcohol, E85, methane, or LP. Alcohol has shortcomings for starting and cold weather. Methane and LP would probably be somewhat linked to gasoline prices and may not be regenerative (sustainable).
What people say they will buy is not necessarily what they will buy. Note the return of the Charger, Challenger, Camaro, and Mustang. All are more fuel-efficient than the original, but hardly high-mileage. Many car commercials portray their cars as 'fun to drive'. Mileage may be important, but for many drivers the more important thing is the image that the car is sporty and sexy.
My husband just got a new car with a diesel engine and he is reporting a huge difference in miles per gallon. Perhaps not as ground-breaking as an alternative power train vehicle, but a start. It was definitely a requirement when considering the next car and my guess is given that he's seeing noteable savings, it will remain a key selling point going forward on any and all new car leases/purchases. Grabbing some mind share in terms of what people consider in terms of requirements is a great start to get some mojo for this new class of vehicles.
Chuck, those are interesting results. I still think that there is lots more efficiency to be had from the internal combustion engine. In Europe there are lots more diesels. They burn clean and get better mileage. About 10 years ago, when I lived in the UK, gas was at least $4 per gallon. I had a car with a 3L engine and an automatic, and I couldn't sell it. Prices were going up at the time. I ended up giving it to the church. I recently heard that manual transmission cars were selling better here. That is what I ran into in England. People generally bought stick shifts.
Lately, I got my car serviced and they put in new spark plugs and injectors (the car has lots of miles). My in city mileage has gone up by about 5 MPG. We can tune the cars we have to run better. On the other hand there is a disturbing trend I see in the US auto industry. They still don't get it. Over the last five years or so, Detroit has been working on getting more power of the same size engine rather than making smaller engines (with better fuel economy) for the same size car. The engine comparable to the one in my car that is available now gets 50 to 75 HP more. That is 20-25% better. They could have increased fuel effeciency by that amount. I think that gasoline cars with 40 to 50 MPG (for a full size car) is doable in the next few years. Will Detroit get there first?
As for what alternative power train I would be interested in, it would have to be a Volt type power train. I believe that is called serial hybrid. The Volt is also plug in. I often go for a while doing only local driving. Now, a Volt running on natural gas would be the best for cost. If there was a real spike in prices, I could modify my habbits (take more public transport, etc.) to almost eliminate gasoline usage with a plug in. On the other hand, the up front costs are so high that the payback take a while with a hybrid. That, of course, depends on the cost of gasoline. Consumption is going down and new sources are being found, so I wonder if we will see those Europe like prices (which are driven by tax policy).
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