Some things stretch gas dollars; some don't. Steve Mazor of Southern California's Automotive Research Center put tips to the test.

MYTH: Oil companies run up prices to cash in on Sunday drivers, so the best time to fill up is Wednesday.

TRUTH: "Really, prices are being dictated regionally, so I don't think that plays a role anymore," Mr. Mazor said.

MYTH: Morning is the best time to pump gas because gas is denser when it is cooler.

TRUTH: It is true that gas is denser when it's colder, but double-walled underground tanks retain the temperature of the gas at the time it was pumped underground. "We sell gas so quickly that if it was hot when it went in, it will be hot when it comes out," Mr. Mazor said.

MYTH: A clean air filter makes a difference.

TRUTH: A few cars had slightly better fuel economy when dirty air filters were replaced, but when cars were tested with blocked air filters, fuel economy "didn't get worse, as we expected," Mr. Mazor said.

MYTH: Gizmos or additives will save gallons.

TRUTH: These products don't help even a little bit, Mr. Mazor said. "We have tested many fuel additives ... or devices that go on the battery cable, and none of them have ever shown any measurable fuel economy improvement."

MYTH: It's better to idle than turn off the engine.

TRUTH: "If you're going to sit for more than about 90 seconds, go ahead and turn it off," he said. It's also a good idea to avoid drive-throughs at a fast food restaurant. "Park your car and go inside," he said. "The longer you idle, the more gas you use."

MYTH: It's always best to sweat it out without air-conditioning.

TRUTH: This is true for vehicles made before 1980, but for modern cars, "it's a relatively small effect" because air conditioning systems have "improved dramatically," Mr. Mazor said. "If you're going below 45 mph, roll the windows down and turn the A/C off. But if you're on the freeway, going faster, roll up your windows and use A/C," he said. The effect could save 5 percent on fuel economy.