Gas station credit cards offer tempting sign-up bonuses, are easy to sign up for on-the-spot, and often offer an ongoing promotion that cuts the cost of filling your fuel tank.

It seems like a gas station credit card that offers a certain amount of money off each gallon of gas you buy with the card would be an easy way to save on an unavoidable expense. If you commute to work and the gas station is one you use frequently anyway, a dedicated card may be a good choice.

For example, the Shell credit card offers $.05 back on every gallon of gasoline you purchase with the card. The sign-up bonus is $.25 back for the first two months you own the card as a new user. If you frequent Shell stations and never carry a balance, the reward rate during the first two months is the equivalent of 10% back. After the introductory period, that rate drops to just 2.14%.

These cards are only for use at a specific chain of gas stations. Like store-specific retail cards, they tend to have high-interest rates that negate the value of the discount or promotion if you carry a balance from month to month.

A more popular option for people who want a gas credit card with a good rewards program is the Costco Anywhere Visa Card. Cardholders get 4% back on gasoline purchases for the first $7,000 they charge on the card. After that, they still get 1% back on gas. Restaurant and travel purchases earn 3% cash back and Costco purchases earn 2% cash back.

For non-Costco members who aren’t interested in joining, another great option for a cash rewards credit card that incentivizes gasoline purchases is the BankAmericard Cash Rewards Credit Card. Cardholders get 3% rewards on gas and 2% on grocery and wholesale club purchases on the first $2,500 of combined charges each quarter. All other purchases get 1% cash back. The sign-up bonus for new users is $150 cash rewards after making $500 in charges on the card within 90 days of opening the account.

When choosing a gas station credit card, it’s important to understand that the average interest rate is high at 23.61%. Unless you are planning an extended road trip, or a long daily commute means you fill up your gas tank more than the average person, a dedicated gas card may not offer a big enough savings to make opening and maintaining the account worth the hassle.

If you decide to get a gas card that’s only good at one chain of stations, make sure you read the fine print. Many of the most attractive introductory offers cap rewards at just $100. Look for any restrictions for redeeming rewards. Some cards offer discounts on certain types of gasoline, only. Others don’t allow you to earn rewards if you pay at the pump instead of going inside and signing for a purchase. A straightforward, everyday cash-back credit card may be a better choice if you want to take the edge off of your gasoline costs.