So What is the Cheapest Day to Gas Up Your Car? Its a question we have been asking ourselves for years---what is the best, and the worst, day to fill my car with gas? Gasbuddy.com has crunched the numbers, and has determined that in San Antonio, motorists will get the best deal on gas if they fill up on a Saturday. "That could be because the markets are closed over the weekend, and oil prices aren't bouncing around," Patrick DeHaan, a senior analyst with Gasbuddy.com, told 1200 WOAI news. Gasbuddy kept an eagle eye on some 140,000 gas stations nationwide and clocked changes in pricing to determine the best price on an hour by hour basis. DeHaan says the best price differs from state to state. In Kansas, for example, you can get the best price on Thursday. In New York it's Friday and in Indiana, it's Monday. "Some states have quite a high percentage of stations which are owned by one retail outlet, and there is a certain strategy coming into play here," he said. He says other factors include times which major refineries ship gasoline to cities across the country, and when large fleets of vehicles gas up. The best day to buy gas also changes from year to year. DeHaan says in Texas, for example, Sunday was the cheapest day in 2010, but it has been Saturday every day since then.Generally, the most expensive day to fill up your car is on Tuesday.