Though many big rigs usually need at least 100 to 300 gallons to fill up, some Colorado fueling stations are forced to ration only 50 to 75 gallons out per truck due to a diesel fuel shortage, according to the Associated Press.The shortage is due to a number of factors, including higher demand from farms during harvest season, problems complying with a new federal clean-air rule and a storm that took out a large Illinois refinery.Starting on June 1, the Environmental Protection Agency ordered all refineries to decrease the sulfur content in diesel production from 500 parts per million to 15 parts per million. The conversion to the new diesel has been disrupted by the need to clean storage tanks along the diesel pipeline system to ensure adherence to the new standard, according to the AP report. The shortages have affected prices at the pump. Wholesale diesel fuel has risen 28 cents per gallon since July 1. The average price of diesel in Colorado was $3.20 Friday and $3.06 nationally, more than for unleaded gasoline, according to AAA.