A few days ago, I posted some numbers on the calculated cost savings of diesel in full-size trucks. Even at the extortionary price of more than $4.00 a gallon, we unscientifically calculated that yes, if you plan on keeping the truck between 11-20 years, you may re-coup your investment in the diesel option.

But that got me to thinking...could it actually be that diesels make more sense on smaller cars? In some cases, it would seem so. For two major reasons: 1. The percentage increase in efficiency over its gasoline counterpart is higher, and...2. The option price is lower.

For argument's sake, I'm going to use the Volkswagen Jetta as an example. The Jetta has long been a favorite of diesel enthusiasts, ever since the first TDI came out in the mid-90s. Volkswagen has announced a 50-state clean TDI will be available for the 2009 model year (already a year later than originally planned.)

The standard gasoline-powered Jetta has a normally-aspirated 2.5L 5-cylinder engine that gets 21 city and 29 highway miles per gallon. Official figures have not yet been released on the new TDI coming out next year, but let's throw out a few speculative figures for comparison purposes. Let's assume the TDI will achieve 34 city and 42 highway. There have been rumors that the highway figure could surpass even 50 mpg for the new TDI, but the 2006 TDI model, under the new 2008 ratings system, achieved 30/38. So considering the vastly more advanced Piezo injection system and advanced engine management, it seems fair to assume a 4-5 mpg improvement. (Not to mention vastly improved power!) So, that gives the gasser an average economy figure of 25, and the TDI a figure of 38 mpg. That's a substantial difference of 13 mpg, or more than 50 percent better, assuming our forecast numbers are correct.

We'll use the same mileage numbers and price averages as for the truck comparison. So that would yield a monthly fuel cost of $182.56 for the gasser, and $149.58 for the diesel. On an annualized basis (52 weeks), that would be a cost of $2,373.28 for the gasser and $1944.53 for the diesel, a difference of $428.75 per year. Let's assume the TDI option is $4,000. That might be a little on the high side, but probably close. Using that calculation, the payoff period for the diesel would be approximately 9.3 years.

If you remember our calculations with the trucks, we figured a payoff period between 11 and 20 years. So you might re-coup your investement on a small diesel quicker, but it will still require some patience.  

LATE-BREAKING UPDATE!!
After a little snooping around online, I came across this thread on the TDIClub Forums, that has a Canadian order sheet for the '09 models. According to this information, the TDI will only carry a $2,000 premium over the regular gas model. If that information also holds true for the U.S., then the potential payoff will only be 4.6 years!