GasBuddy.com says average retail gas prices in Illinois have fallen 7.9 cents in the past week down to $2.26 a gallon. The national average is down 2.4 cents from a week ago and stands at $2.23 a gallon. In Decatur, average gas prices per gallon are at $2.13 and the lowest gas price per gallon in Decatur is $2.02 a gallon. The lowest gas price per gallon in Springfield is $1.92 and in Champaign the lowest price per gallon is $1.97.