Gas prices have dropped 10.1 over the past week in Montana.Gasbuddy.com reports the average price for a gallon of gas is $2.85 as of Monday, Dec. 15, 2025.Gas prices in Montana are 13.9 per gallon lower than a month ago and stan d 8.3 per gallon higher than a year ago.The national average price fell 4.5 over the past week and stands at $2.85 per gallon as of Monday.Nationally, gas prices are 22.0 per gallon lower than a month ago and 12.6 lower than a year ago.The cheapest gas in Montana was $2.60 per gallon on Sunday, while the most expensive was $3.19. ...read more read less