[redpoint5]

Diesel has about 10% more energy per gallon than gasoline, so these engines inherently get better mpg. Considering how common it is to mix ethanol with gasoline, the energy content is even more in favor of diesel (ethanol containing roughly 2/3 the energy of gasoline). On top of that, the diesel engine is more efficient due to higher compression, lower pumping losses, and lower RPM.

The general rule of thumb is that a diesel will get about 30% better mpg than the gasoline counterpart. Just compare the various flavors of Volkswagen Jetta.

Keep in mind, in the US, diesel is currently 8% more expensive than gasoline and buying a diesel model car is more costly than the gasoline counterpart. It would take quite some time to recover that additional expense.

Assuming 12k miles traveled per year, gas @ $3.83 and diesel @ $4.12, with a $4,000 price difference between Jetta gas and Jetta diesel, it would take nearly 13 years to recover the extra expense of the diesel. That doesn't even factor in the interest you could have earned by investing the $4,000 you saved, or didn't make interest payments on. You would break even about the time the car is retired.