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Monthly auto sales show that with gas prices dropping, sales of the Dodge Ram have jumped. - 

Falling gas prices have given American consumers some extra pocket money. So, how are we celebrating? According to one economist, we tend to blow some of it at Starbucks— which - bonus! - means more jobs for baristas.

And it appears that some of us run out and buy trucks and SUVs. New automobile sales reports are out, and Chrysler’s Jeep line and Ram pickup trucks had a great month.

We wondered: Doesn't that seem like kind of a big spend for an impulse buy? Do people really just decide to go for it when gas prices drop? 

"Yeah, that's definitely the case," says Jessica Caldwell, an analyst with the car-shopping site Edmunds.com.  She thinks some of today's SUV buyers may be people who ditched their gas guzzlers in 2008, when the economy tanked and gas prices spiked. 

"They traded into something smaller, they didn’t like it, and they want to go back," she says. "And now that gas prices are low, it gives them that push — or even that excuse — to say, ‘I’m going to have something bigger.’"

So, yes: Car buyers are sensitive to gas prices, and they've got a hair trigger.

And: People often do not do the math on fuel economy. Tom Turrentine, from the University of California at Davis Energy Efficiency Center, interviewed people from 60 households about this very question. He and his colleagues picked bankers, college professors — people he thought would be good at math.

"Nobody knew how much they spent on fuel in a year," he says. "People don't keep track of it." When he asked, they looked confused. 

There was one exception: A guy who built a spreadsheet to figure out the best car for him.  The spreadsheet told him to buy a used Honda Civic.

But he wanted a new Ford Escape, and that's what he bought.  "Despite making all the rational calculations, he went with his heart," says Turrentine.

So — whether we never look at the data, or maybe just ignore it — are we making dumb financial decisions?

Remember, this could work in either direction. When gas prices go up, people don’t just buy more fuel-efficient cars, they pay more for them. MIT energy economist Christopher Knittel wondered: Are they overpaying

Answer: Nope. "What we find is that consumers more or less get it right," he says.

Even without a spreadsheet, the average consumer didn’t pay so much for a Prius that the price increase was more than they could expect to save on gas. 

So: Yes, we’re a little impulsive. But we’re not dumb.

Follow Dan Weissmann at @danweissmann