Example, Box 1: 299 (i.e. Cost of tuning in pounds GBP), Box 2: 15 (i.e. the percentage % gain in fuel economy quoted on our vehicle list), Box 3: 35 (i.e. the miles per gallon you are currently achieving on an average run), Box 4: 138 (i.e. the cost of a litre of petrol or diesel at your local filling station in PENCE GBP). Result of this example comes out as: 12,800 miles. This would be the maximum mileage, as percentage gains are very conservative and normally higher than those quoted in the vehicle lists. On average, a year or less for most drivers!