The increase is especially noticeable in DuPage and Lake counties.
The state's gas tax is going up by a half-cent per gallon to $0.39 a gallon.
WATCH: What to expect in Fourth of July gas prices
Gas Buddy reports prices around Illinois are the highest since 2014, and will likely stay high through the summer.
"I think prices will continue to go up the next few weeks," said Patrick De Haan with Gas Buddy. "Demand will likely peak in late July or early August; I think prices will peak right around that as well."
Right now, the average price around Chicago is $3.45 per gallon.
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DuPage County is doubling its gas tax, from $0.04 per gallon to $0.08 per gallon. It's expected to raise over $16 million more a year for aging roads and bridges.
And a $0.04 per gallon tax hike takes effect Thursday in Lake County, Illinois.
"The state's bankrupt; roads are, you know," Steve Powers said while filling up his tank. "Infrastructure is bad; gas tax hasn't been raised in awhile. I mean, they gotta get their money somewhere."
Illinois has some of the highest gas prices in the nation, which has some drivers rethinking where they'll fill up the next time.
"It's definitely something that is on my mind, and I knew I probably won't fill up all the way here because I'll make it home, make sure I fill up there instead of here," Brody Rude said. "Every little bit counts when we talk about saving money."
All this comes as people hit the road for the holiday weekend.
AAA estimates 2.5 million Illinois residents will be traveling by car, between Thursday and Monday, which could be a good chance to fill up in another state before heading back home.
"If you're headed north to Wisconsin, you'll generally find prices $0.25 to $0.30 lower, northwest Indiana, as well, probably $0.20 to $0.30 lower. So, basically, all the lowest prices are around Chicago, just outside of the city and outside the state," De Haan said.