The average price in Chicago has jumped 46 cents a gallon, or 16%, to $3.27 in the last month. "It really is going to be ugly this week on the West Coast". "Peak gasoline prices may be at best still a few weeks away", GasBuddy says.
The only caveat with the Monday rule appeared to be that prices could tick up a bit by mid afternoon, so the best time to fill up is first thing in the morning - perhaps on your way to work.
There are certain days that are cheaper to fill up than others.
GasBuddy also analyzed GasBuddy foot traffic data in the same timeframe and found that gas stations across the country are least busy on Sundays, followed by Mondays.
The busiest time to get gas is 5 p.m.
The worst day in Pittsburgh and Pennsylvania - Friday.More news: Dirk Nowitzki Announces He's Retiring from NBA After 21 Seasons
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In terms of cost, that's also Fridays.
In the San Luis Obispo area, the average price was $3.86 a gallon on Sunday, the most recent day for which data is available, according to GasBuddy analyst Patrick DeHaan. "We generally see more volatility and higher prices later in the week". But there are other "hacks" to help save you money at the gas pump.
When's the best day to buy gas?
California's already-high gas prices are climbing again, and they're about to hit $4 a gallon - an average price we haven't seen since 2014.
Drivers in the United States are paying more at the pump over the last eight weeks, as retail gasoline prices now averages $2.75 per gallon in the USA, according to GasBuddy, who compiles millions of data points from 135,000 gas stations across the US.
GasBuddy compiles information and rankings on gas prices and convenience stores that sell gas.