TIJUANA, Mexico --- Rising gasoline prices in the U.S. have prompted some drivers to fill up south of the border, where prices can be as much as 25 percent less. As U.S. gasoline prices have skyrocketed, prices in Mexican border towns have remained at about 7.41 pesos a liter, or about $2.60 a gallon. Mexican station owners in Tijuana have seen a surge in business from customers driving cars with California plates, according to a report from the Los Angeles Times. Few Americans are traveling across the border specifically to fill up, but they are topping off more when they travel to Mexico for work or play. Experts, however, warn that Mexican fuel isn't as clean as fuel mandated in California. Mexican fuel has much higher sulfur levels. Not only can it cause greater harm to the environment, it can cause damage to the catalytic converters of the newest cars and trucks sold in the U.S.