A huge difference? What do consider huge? 10% reduction in imported oil?
A 10% reduction would be an amazing start. I don't consider importing oil to be a major mistake ... just so much of it. If, within the next decade, we could reduce oil imports by 25%, it would be a great way to exert some downward pressure on crude prices. Increased domestic production and refining capacity is necessary, too.
It would probably take longer to switch 50% of the personal transportation fleet to diesel anyway. The Japanese cars we know and love, like the Toyota Corolla, etc. etc. are already available as clean diesels, as are most European cars. Imagine the soccer moms getting 40mpg in their minivans. My VW TDI, getting 50mpg ... no problem! My old MB 300td Wagon, 35mpg on the highway ...no problem, except being older tech, it is smoky when cold.
NEWS FLASH! Oil is just wonderful as a source of cheap, clean energy! Diesels for transport. Nuclear for Electricity. Coal for chemicals. Natural Gas for industry and domestic co-generation. All real, all real cool right now tech to get us over the hump to the next generation over the next 50 years.