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I think switching to diesel would be impractical due to refining technology. You can only get a certain fraction of diesel (and gasoline) from oil. Diesel used to be cheaper than gas because there was less demand for the diesel fraction: now it's more expensive due to increased demand.
But in the long term, both diesel and gasoline IC engines are less than optimally efficient. Stirling engines can get significantly more useful energy out of each gallon of fuel than either. Their drawback has been that they are slow to warm up & accelerate, but combine them with a hybrid electric drive and that limit goes away, and the hybrid adds fuel savings from regenerative braking.
In the short term, though, the single change that would produce the greatest savings is to mandate that every car have a real-time fuel consumption gauge :-)
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