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These days, when gasoline isn't expensive, it's going up in price. That makes it easy to concentrate on fuel economy, but it's still best to remember there are plenty of other costs that matter in motoring.

Chances are it's been a few years since you bought a new car. In those years, 40 mpg has become the new 30 mpg, as smaller conventional cars are increasingly able to crack the magic 40 mpg barrier on the highway. And if you consider hybrids, 40 mpg is downright mainstream.

So, should you dump your current ride and buy a 40 mpg car? Sounds like a silly question. Who wouldn't love to pass up the pump on a more frequent basis and save money in the process? Let's consider all the costs and capability trade-offs to see if you'd do better to stick with Old Faithful out in the driveway for a couple more years or choose a new vehicle that gets close to 40 mpg.

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Putting numbers to it
Assumptions may be the paving bricks to perdition, but let's make a couple to get a more accurate view of what 40 mpg means to your wallet. For starters, we'll assume you clock 15,000 miles a year, the high average for U.S. drivers. Let's also say those miles are equally divided between urban and open road driving — half city, half highway. Finally, we have to put a price on a gallon of gasoline; we'll call it $3.75 a gallon for regular and $4.05 for premium.

Next, we need to select a 40 mpg car. One of the most mainstream of the new breed of high-mileage conventional gasoline-engine cars — not a hybrid or electric — is Ford's 2013 Fiesta SE with the aerodynamic Super Fuel Economy package. It's definitely a compact, but it posts an EPA highway rating of 40 mpg and backs that up with a commendable 29 mpg in the city and 33 mpg combined. No need for a charging station in your garage or lugging around heavy batteries: This Fiesta is a $17,000 gas-and-go runabout.

Parking our assumptions and Fiesta next to a calculator, we find that $1,673 goes down the Ford's fuel filler neck in one year. That breaks down to $970 in the city and a bit less on the highway, or $703, which by itself shows the difference between 29 and 40 mpg in terms of dollars — $267 a year.

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The comparative set
Now, let's run some other vehicles through our virtual gas station for one day and see how they compare. Let's say you've been doing the daily shuttle in a Toyota Corolla, an economy car similar to the Fiesta. Fitted with a manual transmission, like the mileage king Ford, the Toyota is rated at 27 mpg city/34 mpg highway. Compared to the 40 mpg Fiesta, the Corolla sounds like a gas guzzler, and indeed the dollars needed to run it are greater: $1,042 for a year in the city, $827 on the highway for an annual total of $1,673.

Do the subtraction, however, and the Toyota costs just $196 more a year in fuel than the Fiesta. If that Corolla is running strongly and has some years left in it, that's hardly enough fuel savings to warrant trading it in and starting a new loan.