Comparison-Shopping More Important When Gas Prices Are Low: Expert

(CBS) -- Despite an unseasonable drop in gas prices, new data suggests you still may be paying too much.

Senior Petroleum Analyst at Gasbuddy.com Patrick DeHaan explains why.

"When gas prices are actually cheaper, the spread between stations dramatically widens," he says. "That means as gas prices are lower, there's a lot of stations charging a low price, but there's more stations charging a high price, too."

Chicago is listed as the U.S. city with the fifth-largest spread in the study, with an average price differential of $0.80.

DeHaan said the difference between getting a truly good deal at the pump versus a phony one here in Chicago can be just a few miles.

"Within one mile, the cheapest price is $2.78," DeHaan says. "Within two miles, it's about the same. But then, magically, if you get within three miles, it drops to $2.28."

The trend of overpaying is expected to continue, so DeHaan urges drivers to beware of prices that seem low, but aren't, compared to others.