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Difficult to isolate the various factors. What we do know is, larger diameter wheels (with the same circumference tire) have the following against them:
1. Heavier
2. Larger spoke/barrel exposed to wind
3. Often come mounted with different (much higher RR) tires
4. Come on different trims of cars, which may be heavier or have worse aero
Ultimately, you could do some testing to try to isolate each of these and tell us what factor truly is the largest. You could do any number of the following:
-Drive two different trim vehicles (Prius with 17" vs 19" for example) which come with different factory tire sizes. The EPA "highway" test has a lot of low speed driving, accelerating and decelerating, and may not be representative of true open road performance.
-Put the 17" wheel/tires onto the vehicle trim that came with 19", and see if the higher trim matches the economy of the lower trim.
-Swap equivalent GT tires onto the 17" wheels, or equivalent 19" eco tires onto the 19" wheels, and compare with the stock tire, and see if the LRR tires make a difference on the highway.
-Compare 17" wheels with 19" wheels when using the same tire series (e.g. eco tires on both).
-Add weight to the 17" wheels to make them weigh the same (or have the same moment of inertia) as the 19" wheels.
-Add some sort of plastic ring to 19" wheels to simulate the smaller aero profile of the 17's.
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My money is on the tire RR being the biggest factor, even at highway speeds, followed by aero, followed by weight - most modern cars have air curtains over the front wheels anyway. I'd like to be surprised, however.
Regardless, we can say for certain that a smaller, lighter wheel with lower rolling resistance tire, has in sum a measurable positive impact on fuel economy.
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