There's no real doubt that diesel engines are far and away the most efficient powerplant to put into vehicles. For instance, simply dropping a 4BT into a Jeep Wrangler doubled its fuel economy. In fact, most manufacturers do make diesel variants of US vehicles for the rest of the world - Wranglers (30mpg), Mini Coopers (60mpg), HiLux (25mpg), and even entire swaths of the Ford product line have export only, diesel variants.

What is holding back these engines such that everyone else can enjoy diesel fuel economy? According to Toyota's Chief Engineer, the United States' self-imposed LEV III standard adds $3000 of emissions equipment per vehicle that simply isn't worth the investment for only the United States. This is, according to Toyota, why there will not be a diesel powered Tacoma in 2016 as rumors had led enthusiasts to believe. A similar comment was once fed back by Mini Cooper engineers, that the added hardware required for DEF was too big for the little car's already packed space. Other manufacturers were on course to produce American diesels but stopped (like the Jeep Liberty) in 2007 when these laws were passed. Now, diesel can generally be obtained only in the higher end models of a product where the company assumes a consumer is already spending big money.

It begs an interesting question. Are the emissions of modern diesels good enough that LEV III standards are too excessive and costing the United States an opportunity to reduce oil dependence even further? After all, the production of diesel is far and away easier than gasoline and can be made from almost any waste petroleum product, too.