In this Money Matters report, Time Warner Cable News’ Tara Lynn Wagner has some tips for those looking to save money on gas by signing up for a credit card that offers rewards for gas purchases.

No one likes emptying their wallet to fill their tank, which is why credit cards that offer rewards for gas purchases are so popular. Some of those cards are affiliated with an actual gas station, but Matt Schulz of says you might want to steer clear.  After analyzing about two dozen gas station credit cards, he says one of the biggest issues was the APR.

 “The average interest rate on these cards is about 24 percent which is a full nine percent higher than your average credit card interest rate, so that's a really big deal and can be really significant,” says Schulz.

Gas station cards and general purpose cards also detour when it comes to rewards. While it is common for general-purpose credit cards to offer three percent cash back on all gas purchases or run temporary promotions offering 5 percent, Schulz says gas station cards just cannot keep up.

“Partly because gas stations only make like two or three cents a gallon off of each gallon of gas, so there's only so much of a discount those folks can afford to give,” he says.

Not that they do not offer discounts, especially at first. 

 “So if you sign up for a card you might get 25, 30 cents off per gallon for 60 or 90 days. and if you are about to take a road trip with the family for spring break or in the summer time, that can be a pretty big savings,” says Schulz.

However, he says once the promotional period is over, the rewards tend to shift into a much lower gear, amounting to only a few bucks a month.

“If you spend about 150 dollars a month in gas you'll end up saving about 6 dollars a month and that's not nothing, but it's not going to change anybody's life either,” says Schulz.

As with all credit cards, when it comes to gas station cards, it is important to read the fine print and understand it. There are ways to navigate the system to your advantage, but you need to know the rules of the road first.    

“Know what the interest rate is, know if there's an annual fee, know any other fees that you would have and you have to pay off the balance every month in full. Otherwise, the math starts to work against you really quickly and you end up not getting anything for those rewards,” Schulz says.