If you take two cars, one with larger wheels(depending on wheel weight and other variables) and one with smaller wheels, then let three different people drive each vehicle how they normally drive. Each driver will have a different MPG, someone who is light on the gas pedal like my mother, she will always have higher MPG than someone like myself or my brother. Your use of the pedal and knowing the sweet spot for the best MPG is more of a factory in your MPGs than tires and wheels... which at the most a 5mpg lost just so your Prius can be more cable Offroad.