CHARLOTTE, N.C. Here's one example of how gas prices can get inside your head.

Sometimes Stacy Boyd drives her 2007 Honda CRV from east Charlotte to South Carolina to visit friends. While she's there, she almost always fills up.

Even if I ve got half a tank, she said, I ll top it off.

Stacy does what a lot of us do: She searches for the lowest price. And when it comes to gas, South Carolina consistently delivers.

On Monday, the average price per gallon was $3.16, the lowest in the nation. That's often the case.A few weeks ago, it was the only state where a gallon of unleaded averaged less than $3. Contrast that with North Carolina, where you'll pay about 20 cents more per gallon (Monday's average price: $3.36).The difference between the two states prices is caused mostly by a 22.4 cent difference in gas taxes, says Tom Crosby of AAA Carolinas.

People constantly cross the border to fill up. You can see that at places like Miller's Produce in Fort Mill, which has added eight gas pumps since Christmas to keep up with demand. Before that, drivers had to get in line.

We'd have them two to three deep waiting on gas, manager Patricia Pasternak said back in January, when gas prices were about the same as they are now. This way, we have the extra pumps to keep traffic flowing easily.

Gas prices are going back up. AAA's Fuel Gauge Report says nationally the price is up eight cents over the last week.

So is it worth your time and money to cross the border to fill up? For the answer, you need to think about two things: math and your mind.

The Dividing Line

If you're north of Charlotte's South End, the drive isn't worth the trouble.

At Monday's prices, the driver of an average car (21.5 miles per gallon, 14 gallon tank) would save$2.80 by filling up an empty tank in South Carolina. But, that same car would burn $2.80 worth of fuel getting there. It s roughly 9.5 miles from the West Boulevard exit on I-77 to the closest South Carolina gas stations on Carowinds Boulevard in York County.

Short answer: If you're in South End, you're spending as much as you're saving to drive to South Carolina for gas. It's a wash.

(A more nuanced answer requires a bit more information, like the gas prices in your neighborhood, the price at a station in South Carolina, the gas mileage your car gets, the size of your gas tank, and the distance you ll be driving. You can calculate your exact savings and find out how we arrived at our answer here.)

NewsChannel 36 ran the numbers for Stacy, who drives 20 miles each way in her CRV to visit friends and top off her tank.

At Monday's price, she d save about $3.06 by filling up her tank in South Carolina, but she d also use $4.68 worth of gas making the trip. So she ends up losing $1.62 on her voyage. She doesn t make trips to South Carolina specifically to fill up, which is good, because she never realized that a trip wouldn t save her all that much money. Instead, she's drawn in by the lower prices she sees.

I d never run the numbers before, she says. I d never thought of that.

The Psychology of Gas Prices

The price of gas can rise and fall, but the gap in the prices between North and South Carolina stays roughly the same, thanks to that 22.4 cent difference in gas taxes. Therefore, the amount you'd save by crossing the border stays roughly the same.

When gas prices in Charlotte hit a record high $4.18 in mid-September 2008, the cheapest average gas prices in South Carolina were $3.95 in the Charleston metro area, according to AAA's Fuel Gauge Report. The prices then were only 23 cents apart. Today, there's a 20 cent difference in the prices.

Bottom line: most drivers can only save a few dollars at most by filling up in South Carolina.

Still, why do so many of us in North Carolina feel compelled to fill up our tanks whenever we cross the border?

Blame the signs, says Dan Ariely,the James B. Duke Professor of Psychology and Behavioral Economics at Duke University.

There s no other category in which the price is printed in such big letters in so many places, he said. Many of us pass gas stations every day. We know exactly what the price is. On top of that, we're often buying multiple gallons at a time, and we see a big price at the pump once we're finished.

All of this makes us make-shift gas price experts; we remember exactly how much it costs to fill up the tank, and we also remember how much it used to cost. Most of us don't have that relationship with other commodities. We wouldn't drive around from store to store, looking for the cheapest prices on different groceries.

I think if we treated yogurt in the same way, said Ariely, we'd also be sensitive to the price of yogurt.

Ariely wrote an op-ed in The New York Times in 2008 in which he argued that we should pay less attention to the price of gas and more attention to the things where prices are actually rising the most: electricity, food and health care.

We re all very sensitive to the price of gas, said Ariely. And because of that, it causes all sorts of bad behaviors, like driving an extra distance just to try and save a few pennies.

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