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I think what the test says is that there is something else going on and you can't compare the rolling resistance of tires of different sizes.
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I believe I've been able to identify at least one of those variables which change in a non-scalar way when tires are scaled up or down for size. For example, the legendary Bridgestone B381 185/70-14 tires have a 5.0" tread width, while the 185/65-14 version of the same tire has a 5.4" tread width. If my math is correct, that's an 8% increase in tread width, which would increase the size of the contact patch and also increase rolling resistance.
While looking at other tires, it seems the tread width of the same tire between different sizes goes up or down almost arbitrarily depending on the brand.
Tirerack is pretty good about giving detailed specs about tires.
JMac