I have read in many posts that you don't get as many MPG with the 17" tires as you do with the 15" tires. I am curious why this is. I understand why the 15" tires give a softer ride (more air in them) but isn't the circumference the same on both tires? If the circumference is the same (which I'm not sure about) why the MPG difference?
Wider contact patch on the road with 17's vs 15's means increased rolling resistance, and slightly less aerodynamic, again due to wider tire and different rims would be my first guess.
It's been discussed in another thread here just in the past week or so. There is nothing inherently different about all larger wheels that give poorer gas mileage. In the case of the stock Toyota offerings, the 17s seem to be heavier overall (when you add the rim and tire) and they have a wider footprint with higher rolling resistance. It could go the other way though, if the larger wheels were lighter and the tires used were a low rolling resistance type.
Bigger rims (and the) tires tend to be heavier. They also tend to have a bigger contact patch resulting in increased rolling resistance, and bigger rims usually have to push more air out of the way. You can mitigate some of these factors with alloy rims, low profile tires, ground effects...etc but those efforts will also give corresponding results in the 15" world. SUV or Prius? Bigger isn't always better.
