It depends. Physics: weight and inertia. The greater the total mass (wheel + tire), the greater the energy needed to move the wheel and car. This of course assumes the vehicle and driver's weight do not change. The only variable is wheel & tire. If the new wheel and tire is heavier than the current wheel and tire, you will have worse MPG, due to greater energy input needed to move the wheel/vehicle. Conversely, if the new wheel and tire is lighter, than the current wheel and tire, you will improve efficiency and MPG. f this does not make sense, imagine putting on a backpack with 20lbs or sand in it. You will expend more energy supporting your new total weight, as you walk around. Loose the backpack, you are 20lbs lighter, and happier, and expend less energy walking around. Base Prius came standard with 15inch wheels. Touring Prius came standard with 16inch wheels. Figure out which wheel size and tire you plan to get. Weight these to figure out the weight. Ideally, you weight the existing wheel with tire dismounted, and weight a new tire that fits the existing wheel for a more accurate comparison. New replacement wheel + new tire for new replacement wheel VS existing wheel w/o tire + new tire for existing wheel Whichever is lighter will give you the better MPG. I would have to speculate a large weight savings would be required to have a meaningful MPG increase compared to the current wheel/tire on the car. There are some really nice aftermarket wheels that are very light, but these command a hefty price. Also, the tire you get and PSI used can have an immediate impact on fuel efficiency irregardless of wheel size and wheel weight. LRR (Low Rolling Resistance) tires vs non-LRR tires; LRR supposedly gives you the best MPG.
Size is not as important as the rolling resistance of the tire chosen. You can have inferior or superior choices in any of the available sizes.
I agree with 05.. The god awful integrities got great mileage but sidewall hard as a rock. LRR tires are hard.
