How wheel & tire size and style affect Mirage fuel economy/mileage

[MetroMPG]

In New Zealand, just two Mirage models are offered: LS (base) and GLS, both with the CVT automatic transmission only.

They both have identical drivetrain/technical specifications, yet the LS has a better fuel economy rating than the GLS, burning 6.1% less fuel. Why? Mitsu points to the wheel/tire spec on the two cars as the culprit.

N.Z. AA explains:

the difference is a result of the variance in wheel sizes. Skinny 14-inch steel wheels are fitted to the LS models while the GLS versions are fitted with 15-inch alloy-wheel rims with slightly wider tyres.

http://mirageforum.com/forum/showthr...e-Association)

(Off-topic... I think it's funny that they call the 165 width tire "skinny", but not the tire that's only 1cm wider. Duh.)

	Mirage LS 1.2 L CVT automatic	Mirage GLS 1.2 L CVT automatic
Combined fuel economy:	4.6 L/100 km 21.7 km/L 51.1 mpg (US)	4.9 L/100 km 20.4 km/L 48 mpg (US)
Wheel & tire	14" Steel w/ Wheel Cover 165/65R14	15" Alloy 175/55R15

How do different wheel /tire size and style cause the GLS to get worse fuel consumption?

• the slightly narrower tire will have a slightly better drag coefficient and smaller frontal area
• that flat plastic 14 inch wheel cover is probably more aerodynamic than the open spoke alloy wheel
• the different tires probably have different rolling resistance coefficients
• the bigger alloy wheels & lower profile tires may have more of their mass concentrated further away from the rotation axis, which would need more power to accelerate

I haven't compared the overall diameter of the 2 tires, but I assume they're very close. If they're not, though, gearing could also be slightly affected.

UPDATE (Jul 11/2013):

Mitsubishi again points at wheel/tire choice and its effect on fuel consumption, this time regarding the new Mirage Sedan G4 / Attrage:

22 KM/LITER. (with 14 inch wheels) Having the lightest body weight, it isn’t surprising that the Mirage G4 has the lowest fuel consumption in its class. [...] Equipped with 15-inch wheels, the Mirage G4’s fuel efficiency is rated at 21 kpl.

source: http://business.inquirer.net/131241/...ry-chapter-two

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FYI: here's a dramatic example of how these wheel size/style issues can affect fuel economy:

• Prius 15's are better than aftermarket 17's! (Wrong wheels/tires can wreck your MPG)