Larger wheels are heavier and have a larger turn diameter (not sure if that's the right word, maybe "ratio"??) so that lowers your fuel economy. I went from 17" wheels, OEM, on a Hyundai Sante Fe Sport AWD 2.4L, to 19s and the MPG went doooooown. From 24 to between 17-21, it was all over the place, couldn't get an accurate read after changing the wheels. That sucked. But my CX-5 has 19s with the grand touring pkg, and the fuel economy is great. Avg 25. Think it might be the Skyactiv engine. Mazda has always had good fuel efficient engines, don't think Hyundai can compare.

 

Wheels diameter:

• The weight of the rim component of any wheel is what affects the rotational mass (aka. angular kinetic energy) the most.
• The further the rim is from the center of rotation, the greater the rotational mass, at any given RPM, even if the weight of two different wheels happens to be the same, the wheel with the larger diameter will have a higher rotational mass.
• The diameter of a wheel has very little to do with the overall diameter/circumference/rotations per mile (I assume this is what KPTX meant by "larger turn diameter") of the wheel/tire assembly; tires are the component which determine this aspect. Said another way, 205/65 R15 tires mounted on 15" rims will have a larger diameter (25.4") than say 225/45 R17 tires mounted on 17" rims (diameter 25.0").
• The reason why larger wheels (in addition to their other negative performance impacts) hurts fuel economy is because the greater the rotational mass of the wheels, the the greater the energy required to accelerate them up to speed.
• The converse is also true in that a car fitted with say 16" wheels from the factory will stop quicker than the same car fitted with say 18" wheels.

 

Assuming you use a tire size that maintains the same overall diameter when going to a larger diameter wheel, the only thing that could negatively impact fuel economy is wheel weight. That is a function of design and materials and it's not difficult to find 19" rims that weigh the same or less than 17s. That's why Mazda lists the same fuel economy ratings for the Touring (17") and GT (19"). The rest of the story is all about quicker turn-in, potentially better handling and possible effects on ride quality.

 

Not true, but not far off either. The fact is, if you have two wheels with both weigh say, 25 pounds, one 17" in diameter and one 19" in diameter, even if the tires mounted on them have the same overall diameter, the 17" wheels will have a lower rotational mass than the 19" wheels, and as such, will take less energy to accelerate up to speed. How much different? Not all that much (hence the "not far off either" comment), the difference in rotational mass between 17" and 19" for wheels of the same weight is fairly small, and probably not enough to amount to even 0.25 mpg.

 

I'll accept that!