Need to fill up the gas tank? If you live in South Dakota, you might want to wait until Wednesday.

Midweek is the cheapest day to fill up in the Mount Rushmore State, according to a recent study by Gasbuddy.com.

Taking a weekend trip to Minneapolis, Minn.? You’re in luck. GasBuddy says the weekend is the cheapest time to purchase gasoline in the Gopher State.

Weekends are the cheapest for most states, GasBuddy found, but some states like South Dakota had lowest prices on weekdays.

In fact, over the course of the study, South Dakota was also one of the few states to have a consistent day of the week from 2010 to 2013—Wednesday—to purchase gasoline. Wednesday was the cheapest day in 2010-12, with Saturday breaking the trend in 2013.

The best day of the week to purchase gasoline in other Northern Plains states according to GasBuddy: Iowa, Monday; Montana, late week; Nebraska, late weekend; North Dakota, mid-week and Wyoming, Tuesday and Thursday.