I'm impressed that many of my classes seem to know the fuel economy of a car in miles per gallon. Problem is that petrol is sold in litres now. As part of my quest to verify the data in David MacKay's book last summer, I started noting the amount of fuel that went into the car and the mileage. Over 5 weeks of the summer holiday, our car averaged about 17 miles per litre. Looking in the handbook, that comes between the given values for motorways and for city driving, so it sounds about right. They test their cars on rollers in a lab so I'd like to know if they factor in air resistance as well.