13 gallons left.
Determine the total number of gallons in a full tank by dividing the total range by the average mileage per gallon: 675 miles \/ 45 miles per gallon = 15 gallons. Subtract the amount of gas already used from the full tank. Since Luanne has used 2 gallons, she has 15 gallons - 2 gallons = 13 gallons left.