Iowans are paying more to fill up their gas tanks this month. The monthly survey from the state Department of Natural Resources finds gasoline is costing Iowans, on average, a dollar-40 a gallon in this month. That’s up 12-cents a gallon from July and it’s a nickel higher than the national average. The D-N-R says refinery problems in Illinois, Ohio and Texas are putting more stress on already-tight Midwest gasoline markets. Diesel prices in Iowa dropped a penny in the past month, now averaging a dollar-45 a gallon. Propane prices also dropped since July, about four cents, to a statewide average of 83-cents a gallon. Heating oil prices have also fallen two-cents a gallon to a dollar-six though they’ll head back up soon as demand rises.