The Crittenden County school district in Kentucky expects to save thousands of dollars a year by switching to a propane powered school bus, the Park City Daily News reported. The propane-powered bus averages 4.63 miles per gallon, compared to the 6.82 miles per gallon that diesel buses average. However, propane is considerably less expensive than diesel. Diesel costs $0.55 a mile, while propane costs $0.22.