My cousin in law Dave has been complaining to me about our 2007 Kia Sportage getting bad gas mileage. He says it's been getting 19 mpg, but when I drive it, it gets 25.5 mpg! We both drive very conservatively, and we drive on the freeway mostly. I'm on the freeway more often, and I'm very tall and thin like a telephone pole, while Dave weighs enough, last summer, he broke one of our dinning room chairs. Can being fat cause worse gas mileage?