Just a month ago, filling a tank in Minnesota left a big hole in your wallet. Average gas prices in the Twin Cities peaked at $4.35 on May 18. By late last week, though, the average price had dropped to $3.52, lower than the national average, according to GasBuddy.com.

Time to pack the trunk and rev the engine.

Road-tripping to Brainerd from the Twin Cities? That'll be $37, assuming your car gets the national average of 25 miles per gallon. Bound for Ely? Make it $69. Park Rapids? $55. Now is very good time to explore the state.

Do the math as I've done, and you'll see that the true difference in gas prices is insignificant in the context of a family vacation budget. The trip to Ely, for instance, would have cost $86 in mid-May. Drive there today instead and the savings would be about the cost of one meal out, but still, gas prices above $4 a gallon are pretty tough to swallow (and who wouldn't prefer a delicious sandwich?).

Why did prices spike in May? Chalk it up to the law of supply and demand.

Less gasoline was available in the Upper Midwest that month, when two oil refineries in the region underwent maintenance. Predicting gas hikes is a tough business, since refiners generally don't share their maintenance schedules.

"Could it happen again? Yes, but it is not very likely," said Patrick DeHaan, petroleum analyst for GasBuddy.com. For the time being, anyway, the refineries that had stopped operations are back online, and the gasoline is flowing.

Let it flow, let it flow, let it flow — from the refineries to gas stations across Minnesota and, finally, to your gas tank. After all, it's summer, and time to take that road trip Up North.

Send your questions or tips to travel editor Kerri Westenberg at travel@startribune.com, and follow her on twitter @kerriwestenberg.