Don't dawdle using up the gas discounts you earned at the grocery store.

Giant Eagle this month tightened its rules and set a shorter expiration period for rewards earned through its fuelperks! program. Shop 'n Save and Foodland programs already had shorter cutoff dates for their gas discounts.

All three programs have been around for several years and have gained fans among shoppers. Although the details of each offering differs, essentially, customers who sign up for grocery loyalty cards earn fuel credits when they buy groceries. And the more groceries they buy, the more discounts they build up. People enjoy talking about getting a free tank of gas or at least a price much lower than the $2.60- to $2.70-a-gallon rates found around the region.

Effective July 1, Giant Eagle moved its expiration date from what had been three months after the last day of the month that the rewards were earned to two months after the last day of the month in which they're earned, company spokesman Dick Roberts explained in an e-mail.

While that's less time than before, he pointed out, "Depending on when fuelperks! rewards are earned during the month, customers may have up to 90 days to redeem their fuelperks!" For example, someone who picked up groceries on July 2 would have all of August and September to use the gas discounts.

Mr. Roberts said the majority of customers use their discounts within 60 days of earning them.

Shop 'n Save's Pump Perks program is set so the discounts earned "expire 60 days from the date of issuance," according to the grocer's website. Gas rewards offered through Foodland's program "expire at the end of the month following the month in which they were earned," according to the description posted online.

There typically are sound reasons to offer shorter expiration dates for any promotion, said Jim Hertel, senior vice president of consulting firm Willard Bishop in Barrington, Ill. For one thing, it can focus shopping activity into a shorter time period. A coupon that expires in one month versus one that is good for three months will push any related sales into that smaller time period.

"There's a gentle nudge," said Mr. Hertel.

A shorter promotional period also can limit the company's exposure to the cost, he said. Although he wasn't specifically familiar with the Giant Eagle offering, he said a one month change probably wouldn't have a significant impact as, say, trimming an expiration period by six months.

He offered the example of the airline industry, in which some frequent flyer programs once had few limits on how long miles could be accumulated. Now expiration rules tend to be more strict.

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