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People tend to grouse about gas stations jacking up their prices on weekends, but in most states, gas prices are higher during the workweek.

The crowdsourced gas-price aggregator took an exhaustive, four-year look at what days gas was cheapest in each of the 50 states and Washington, D.C. While gas prices obviously fluctuated during that time period, senior petroleum analyst Patrick DeHaan said the results showed that prices were lowest on Friday, Saturday or Sunday in about two-thirds of states.

“I think that was the biggest surprise,” DeHaan said. “What I can surmise is that the markets aren’t open, so because oil and gas futures and spot prices aren’t trading on the weekend, stations are more likely to become competitive,” he said.

Fill 'er up, please, only do it on the weekend when it's cheaper in most states.Danny Johnston / AP

Since people drive more outside the scope of their typical Monday-to-Friday routine on the weekends, gas station owners want to take advantage of any additional traffic that comes their way. On weekends, drivers also are more likely to have time to comparison-shop, which also encourages station owners to compete more on price.

There are a few regional anomalies: GasBuddy’s analysis found that Western states, including the Dakotas, Wyoming and Utah, tend to have cheaper gas in the middle of the week, as does Kansas. Monday is usually the cheapest day to fill up in Indiana, Iowa and Kentucky, while Maryland and Delaware’s least-expensive day is Thursday.

Finding the cheapest gas anywhere can be a moving target, though. “The patterns certainly are evolving over time,” DeHaan said. “There’s certainly a possibility that the trend could be shifting.”