CIUDAD ACUNA, Coahuila, Mexico -- People going across the border to save some bucks on gas may find out the hard way about a new Mexican law.

Video: Couples Loses Truck At Border

The new law restricts the amount of gas Americans can buy across the border, and one couple told San Antonio television station KSAT that Mexican authorities seized their vehicle and won't return it for at least four days.

"If it's a law, why isn't there a sign for American people to say, 'We're limited on diesel'?" said Andy McCulley in a telephone interview from Mexico. "Had we known that, we would have never, never done this."

McCulley said she and her husband have been going across the border into Ciudad Acuna to purchase cheap gas for decades and never had a problem until Thursday afternoon. As they entered the customs area, McCulley said, Mexican authorities asked if they'd purchased any diesel. After responding yes, McCulley said, their 2006 GMC pickup was impounded.

The price of Mexican diesel fuel is about half the cost of diesel in the U.S. Mexican authorities said the new law is designed to prevent bootleggers from making a profit.

Bobby McCulley admitted to authorities they had put 10 gallons in the truck's tank and an additional 50 gallons into an auxiliary tank. That's what triggered the law.

Mexican authorities told the McCulleys they would need a permit to purchase that amount of fuel, sign some papers and leave the truck in Mexico, McCulley said. They refused.

"They won't let us get near the vehicle, they've taken the keys away (and) they finally gave my husband back his driver's license, but they won't give us back the keys," Andy McCulley said.

The McCulleys said they've encountered other Americans who have also had their vehicles seized, but the McCulleys said they were told they couldn't have their truck back for the time being.

"This is uncalled for," Andy McCulley said. "It really and truly is uncalled for, and I will stay here. I will get some deodorant, but I will stay here."

Val Verde County Sheriff D'Wayne Jernigan and Del Rio Mayor Efrain Valdez confirmed Mexican customs officials were stopping trucks with auxiliary tanks not installed by the manufacturer. The seized vehicles are being held for about two weeks, according to county sources.

Ciudad Acuna officials said they concerned about the possible impact to local tourism if the law continues as stated and have been working with U.S., Val Verde county and Del Rio officials to resolve the McCulleys' and others plights at the border.

The McCulleys have offered to return the gas in exchange for their truck as well as pay any fines for violating the law, but Mexican officials have not taken them up on their offer.