Since the second half of 2007, the price of diesel in the U.S. has climbed from its historic level of selling at or below the price of regular gas to a point 15-to-20 percent higher than premium for much of 2008.

For automakers who have been planning on introducing new 50-state legal diesel vehicles in 2008-09 – as well as drivers looking forward to buying them – this has been a big problem. The price differential ate up much of the operating cost savings that many pricier diesel vehicles were still able to provide thanks to their greater efficiency. Even as gas prices dropped from their highs of $4 per gallon last summer to less than $2, diesel remained stubbornly expensive.

That situation has finally changed in recent weeks, though. On a trip to northern Michigan a couple of weeks ago, we started seeing stations actually selling diesel for less than regular gas for the first time in two years. That same scenario has now migrated to the Ann Arbor area, where we shot the sign to the right recently.

Nationwide, diesel is now averaging just under mid-grade gas at $2.023, while regular is at $1.910. In fact, according to the Energy Information Agency, the average price of diesel has plunged over 20 cents per gallon in the last six weeks, while the price of the various grades of gasoline has more-or-less held steady, keeping prices within one or two cents. Only time will tell if this is part of a larger trend or just a temporary dip.

[Source: Energy Information Agency]