(MONEY Magazine) – The last weekend in April, David Landay pulled his Volvo wagon up to the pump at the Elmesco ATI service station in Cold Spring, N.Y. The attendant--it's a full-service station--sauntered up to Landay's car, motioned him to roll down the window and told him matter-of-factly, "We're out of regular."

Landay couldn't believe his ears. How could a gas station run out of regular? Easily. In fact, says Kenny Elmes, the station's owner, it has already happened to him more than once this season.

"About 25% of my customers who used to buy mid-grade or premium have switched over to regular," he says. That's precisely what happened when gas prices leaped last summer. According to the National Energy Information Center, sales of premium gas dropped 25% from July '99 to July 2000, while sales of regular gas rose by 5%.

What's going on here? A little mental accounting. It seems that we interact differently with our cars when we find ourselves paying more for gas. Some of our new behaviors make sense, but others could be risky. Here's a rundown.

Buying regular instead of premium. Verdict: A fine move. Unless your car's engine was made to run on the pricier stuff--and only 20% of the country's cars are--buying premium is a waste of money, says George Littell, an energy consultant in Houston. How much money? A driver who logs 15,000 miles annually in a car that gets 25 mpg and buys high test would be out an extra $120 a year. The tab could be double for SUV drivers. The exception: Older cars tend to operate better on higher-octane gas.

Delaying regular maintenance. Verdict: No good. "It's not easy to reduce your gas consumption, so people put off other spending that's more discretionary," says Geoff Sundstrom, spokesman for the national office of AAA. His big worry: maintenance. You might not want to spend $40 to change your transmission fluid, but if you don't do it, you may find yourself shelling out $1,000 for a transmission overhaul.

Shopping around for a better gas deal. Verdict: Why not? The range in gas prices from one station to another within your community can add up to big savings. Take Dayton. On May 8, the low price in town for regular gas was $1.68; the high, $1.89--a difference of more than $4 for a single 20-gallon tank. To find the best prices in your zip code, surf to www.gaspricewatch.com. If you're wondering whether buying from a grocery store or no-name independent ups your risk of putting bad gas into your car, the answer seems to be no. The real question: Are you driving out of your way to purchase it?

Not filling up your tank. Verdict: Costs time and money. Putting in $20 worth because that's what you used to pay is mental accounting run amok. "When your gas is running low," says Kenny Elmes, "your car is running on [gas from] the bottom of the tank where all the sediment sits. That's not good for performance." And what if prices are even higher next week?

The bottom line. This is the time for some good old conservationist behavior. Leave your SUV in the driveway; let your Honda do the commuting. And combine those errands--short trips add up. According to Sundstrom, the high prices we're seeing this summer may be with us for the foreseeable future.

Editor-at-large Jean Chatzky appears regularly on NBC's Today. You can contact her by e-mail at moneytalk@moneymail.com.