Let's use good old maths to work out whether the TDI is a good deal.

Let's just say that a VW TDI costs $15000, and gets 50MPG, and compared to a small hatchback that costs $10,000 for similar age/miles, and gets 40MPG.

Using

these figures, we'll assume that gas is $2.66 a gallon and diesel is $2.85 a gallon. We'll also assume that you drive to work (80mi RT) for 240 days a year, and drive an average of 20mi per day for the other 125.

As a result, the estimate of total miles driven per year would be (240 x 80) + (125 x 20) = 21700 miles per year.

In the VW, 21700 miles would use a grand total of 434 gallons of diesel, at a total cost of $1236.90 for a year. In the small hatchback, you'd use 542.5 gallons of gas, which would be $1443.05 for the year. If gas was $4 and diesel $4.50 a gallon, the VW would be $1953 and the small car would be $2170.

So, you're saving roughly $200 a year on fuel, go for the VW right? Wrong. After all, you spent another $5000 to buy the thing in the first place. That $5000 is no longer working to generate income for you. Based on a 7% rate of investment growth, you've forgone $350pa in investment returns to save $200pa in gas.

Of course this doesn't take into account things like insurance, servicing, expected lifespan, or similar. YMMV :)