With gas prices rising, people are beginning to talk about fuel economy again. This year, there’s an added wrinkle to my own concerns. In April, I ditched my 2000 Ford Focus for a 2004 Mini Cooper. The old car used regular unleaded, but the new one uses premium. This is the first vehicle I’ve owned to use premium gasoline, and the price difference came as a shock to me.

But am I really paying more for fuel? Fortunately, there’s an easy way to check.

Last August, I began to use Fuelly, a single-purpose web application. All Fuelly does is let you track your fuel economy. That’s all it needs to do. I’ve been a devoted user for 10 months now, entering all of my stats at every fuel-up.

As a result, I have enough data to determine whether my Mini Cooper is picking my pocket for fuel. Here are the summaries for our three vehicles:

Note that the average fuel economy on my Mini Cooper is better than the best tank on my Ford Focus! (I wonder if some of this is due to the fact that the Mini has a real-time fuel consumption calculator. I use it all the time, and have become very aware of the kinds of things that help or hinder my fuel economy.)

The Focus averaged 25.7 miles per gallon. According to GasBuddy, the gas station I use most often is currently charging $2.61 per gallon for regular unleaded. At this price, I would use 10.16 cents worth of gasoline to travel a mile in the Focus.

My new Mini Cooper, on the other hand, is averaging 29.8 miles per gallon. Its premium fuel at the same gas station costs $2.85 per gallon. That means a mile of travel in the Mini uses 9.56 cents in fuel.

It would be nice if my car used regular unleaded, but it’s comforting to know that even though I’m paying more per gallon, I’m not actually paying more per mile.

Sidebar: One small but important detail: The Focus averaged 265.5 miles between fuel-ups. The Mini averages 373.3 miles. It may seem silly, but I like not having to visit the gas station as often.

Other personal-finance bloggers have come to love Fuelly, too. At his blog, MyLifeROI writes that for him, the ability to compare his mileage with other Fuelly drivers helped him learn that he had some bad habits:

After using the site for a while I realized my driving habits were causing my mpg to be about 3 less than the average across the site. I decided to start driving more consciously and since then I have brought myself up to 1 mpg better than the average! For myself, at an average of 1,000 miles per month, what does this mean for me? A savings of 4.5 gallons per month, or about $10. It also means I am most likely driving my car more sanely and decreasing the wear on my engine and transmission.

Fuelly is a great single-purpose financial website. Can you recommend others? I don’t mean financial managment applications like Wesabe and Mint, but focused tools like Fuelly or Zillow or PriceProtectr. I’d love to compile a list for a future post.

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