We bought our 2010 Volkswagen Jetta TDI wagon in November 2009 in large part based on the potential money we’d save at the pump. In the wake of the current diesel emissions fiasco and the fact that until recently, diesel has consistently been more expensive than gasoline, last night Julie? asked if we’ve even managed to save any money with this car.

I went to the Energy Information Administration website (an outstanding resource for historical data on fuel prices and supply) and downloaded the monthly average price data for the midwest which is available in a handy excel spreadsheet. You can also get national and regional data for annual and weekly averages. Based on the total mileage accumulated over 70 months and the EPA combined fuel economy estimates of 34 mpg for the diesel and 25 mpg for the 2.5-liter five-cylinder that was also available at the time I worked out the average monthly fuel consumption over the time we’ve owned the car. We have used approximately 23.1 gallons of ultra-low sulfur diesel per month and would have used about 31.4 gallons of regular gas with the five cylinder.

Putting that up against the average monthly price of regular gas and diesel we would have spent approximately $7,274 on gasoline. By comparison, despite the higher pump prices most of the time, I estimate that we’ve spent roughly $5,732 on diesel fuel, a savings about $1,540 dollars. The diesel emitted approximately 16.3 tonnes of CO2 over the past six years while the gasoline-fueled version would have output almost 19.4 tonnes, so we’ve saved roughly three tonnes of CO2 as well.

Does that justify all the extra NOx we’ve emitted? I have no idea. That’s one of those trade-offs that you simply can’t measure directly. Having paid a roughly $2,000 premium up front to buy the diesel engine, the economics are still an open question right now. A week ago, the $3,500 in extra resale value of a diesel compared to a gas engine would have more than covered the $500 difference we’re at right now in fuel savings and we would have been way ahead financially. A few weeks from now when this all sinks in, who knows what this car will be worth.

I don’t have answers yet, just some random data points. Stay tuned.