I figure that my family has saved at least $700 on gasoline over the past 18 months by aggressively taking advantage of grocery store promotions.

My wife and I each drive about 900 miles a month, which is about average for most people, and we have fuel-efficient vehicles, so we don’t use a great deal of gas. But when we do buy gas, we typically pay 20 percent less than the price at the pump, and those savings add up fast.

Many large grocery store chains, and the CVS drugstore chain, frequently offer discount deals related to gasoline.

Maybe gasoline’s not the first thing you think of when you go shopping for groceries, but you may be missing out if you aren’t reading the store circulars and looking for deals.

The downside is that some of the promotions are unnecessarily complicated.

Here are some of the deals in the Charleston area, and how they stack up:

Bi-Lo: The Fuel Perks promotion allows members of Bi-Lo’s BonusCard loyalty program to save 5 cents per gallon on up to 20 gallons of gas, at a participating gas station, for every $50 spent at Bi-Lo on groceries, with some restrictions. Purchasing promoted items can generate additional Fuel Perks, which expire in 91 to 120 days.

What’s good: Fuel Perks reduce the price-per-gallon right at the gas pump. You don’t have to buy gas cards.

What’s bad: Fuel Perks offer a per-gallon discount for a single transaction, up to 20 gallons, so the potential for savings depends on the size of a vehicle’s gas tank, and how full it is when fueling up.

Bottom line: To save $5 on the maximum 20-gallon gas purchase, someone would have had to spend at least $250 on groceries. It amounts to getting up to 2 percent back on grocery spending, or half that much if you rarely buy more than 10 gallons of gas at a time.

CVS: The drug store chain frequently offers a promotion during summer months in which customers who buy $30 of selected items get a coupon for a free $10 gas card available at the store. Store circulars advertise the details.

What’s good: Getting $10 back on a $30 purchase.

What’s bad: Having to buy selected items, which may be available for a lower price elsewhere.

Bottom line: If you need the selected items and they’re being offered at a fair price, then spending $30 and getting a $10 gas card back is a very good deal. Think of it as a one-third-off sale on the items you have to buy, or 33 percent cash back.

Harris Teeter: Customers with a VIC Card can buy a $25 Shell or BP gas card for $20, every time they spend $40 in a single visit. The cards must be purchased in a separate transaction, but don’t they have to be purchased in the same visit.

What’s good: Spending $40 in a single visit is a low threshold for earning a $5 discount.

What’s bad: Having to go to the customer service counter and purchase the cards one at a time, even when buying multiple cards.

Bottom line: Save $5 for buying at least $40 in groceries and a $25 gas card. That’s a 20 percent savings on gas and a total savings on the groceries and gas card of up to 7.7 percent.

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Piggly Wiggly: Customers with a Pig Card may earn Greenbax when they make purchases. Greenbax can be redeemed for discounted gas cards, or used to pay for gas at participating stations. Five Greenbax is worth $5 off a $25 gas card; one Greenbax equals $1 toward fuel at participating gas stations.

What’s good: With one Greenbax equal to $1 in savings, it’s clear what customers save.

What’s bad: Complicated earning rules. You only get Greenbax at The Pig if you spent at least $100 there the previous month. Depending on prior-month spending, a customer might receive anywhere from zero Greenbax to triple Greenbax for purchases.

Bottom line: A qualifying customer would need to spend between $33 and $100 to earn a single Greenbax worth $1. That amounts to cash back worth between 1 and 3 percent. Store promotions may give customers extra Greenbax for purchasing particular items or using coupons.

Publix: From time to time, Publix runs a special where customers who spend $50 on groceries can buy a $50 gas card for $40.

What’s good: It’s a straightforward deal that saves $10 each time. At Publix, when the deal is offered, customers can buy a gas card with their groceries in a single transaction. No loyalty card is required.

What’s bad: The promotion is offered only occasionally. Customers used to have spend just $25 to get the same deal.

Bottom line: Save $10 for buying $50 or more in groceries and a $50 gas card. That’s a total savings of up to 10 percent.

Saving money on fuel is a real savings, because it’s something most people have to buy. Having to buy particular things in order to get fuel promotions tends to whittle away at the savings. But when you can get fuel discounts for simply shopping as you normally would, that’s free money.

Reach David Slade at 937-5552 or Twitter @DSladeNews.