The peak time when gas stations are busiest: 5 p.m. Friday. (Dreamstime)

Gas prices seem to fluctuate each week without warning. Fuel prices are up 50 cents since the beginning of the year, and spike even higher during peak travel times, such as Memorial Day and summer vacations.

But you can fight back by filling up your tank on the right day, according to a new survey by GasBuddy.

The fuel-savings app tracked prices in 30 cities from January through March to find the best and worst days of the week to buy gas. GasBuddy lists real-time fuel prices at more than 150,000 gas stations and convenience stores.

Friday was the worst day to fill the tank, the study found. The peak time when gas stations are busiest: 5 p.m. Friday. Saturday and Sunday weren’t much better.

Save the most by filling up from 7 to 10 a.m. on Mondays and Tuesdays, the survey said. Wait times are the best then, too.

Wednesdays and Thursdays were good days to top off the tank, but not the best.

“While filling up on a Sunday can save time, it doesn’t save money,” says Patrick DeHaan, head of petroleum analysis at GasBuddy. “Our data shows that filling up on a Monday morning each week can collectively save drivers $2.1 billion and avoid the possibility of playing bumper cars at the pumps.”

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