At what $/gallon would it slow you down?

I fully expect energy prices to double or triple over the next decade based on world demographics and economic growth. Those billions of people in the developing world are buying cars and air conditioners and everything else that we enjoy in the developed world, just like we did as our economies developed back in the 20th century. At current fuel prices, the up charge to go for diesel over gas is about even (balancing up front cost vs fuel savings and slower depreciation). If one recalculates based on expected future higher fuel costs over the next 10+ years, the investment in a more fuel efficient vehicle is easily cost justified.