Some cars get better mileage than others. Maybe one is a newer model. Maybe one’s got a clogged carburetor. To say that a car is a car is a car because they all have engines is asinine. Heck, two cars coming off the same assembly line could be quite different. You couldn’t tell just by watching them drive by. Metabolism is affected by a thousand different factors. If anything, the simple statement ‘calories in calories out’ is an over generalization that just make your pedestal look cheap.