Best miles is the higher miles per gallon. Divide miles by gallons to find miles per gallon:
Mrs. Walters : 360 miles / 25 gallons = 14.4 miles per gallon.
Mr. Walters: 480 miles / 32 gallons = 15 miles per gallon.
Mr. Walters can drive further per gallon so he has the better mileage per gallon.