Drivers heading out of state over the Memorial Day weekend are best off waiting until they are far outside Chicago and its $3 plus a gallon average gas prices to fill up their tanks, one expert advised Wednesday.

Average prices for regular gas on Wednesday were 50 cents cheaper per gallon in Wisconsin and Indiana than in Chicago, where prices climbed to $3.158, the AAA’s Fuel Gauge Report showed. Michigan’s average gas price Wednesday was 46 cents less than Chicago’s.

Motorists are better off buying gas “anywhere but Chicago. That’s kind of the golden rule,” said Patrick DeHaan, senior petroleum analyst at gasbuddy.com. “Get at least 100 miles away from Chicago or Milwaukee.”

DeHaan predicted prices around the Great Lakes that have not already spiked could likely see increases of between 5 cents and 15 cents a gallon by Memorial Day — and even a 20 cent jump in some isolated cases.

The cost of a reformulated Chicago summer blend gas is being affected by constrained supply because of weekend problems at an Exxon Mobile refinery in Joliet, DeHann said. Such gas is used in counties bordering Lake Michigan from Milwaukee through Chicago and into northwest Indiana, he said.

Meanwhile, Illinois Tollway officials predicted a record 7.4 million vehicles would travel the state’s tollway system Friday through Tuesday for the Memorial Day holiday. That’s a 4 percent increase from last year, officials said.

Friday is expected to be the heaviest travel day, with nearly 1.9 million vehicles on the tollways compared with 1.5 million on an average day.