Some thoughts on the chart I provided:
It's from a presentation made by Smithers, an independent tire testing company based in Akron, OH, to the California Energy Commission. Here's the whole presentation:
http://www.energy.ca.gov/transportat...%20Testing.pdf
This was part of the CEC's investigation towards writing a tire regulation, with the idea of mandating that only tires with low rolling resistance were to be sold in California.
It doesn't say in the presentation, but I learned that ALL the tires in the tire size study were Goodyear Integrity's (not sure where I picked that up), but it makes sense in context. By looking at an entire line of virtually identical tires, the CEC could understand what they were faced with in trying to write a regulation.
They also didn't state what test they performed, but in order for the chart to have any meaning, the test had to be the same. Please note, that there are standard RR tests that correlate very well to steady state energy consumption in a tire - and because it is a lab test, the conditions can be closely controlled to eliminate things that a coastdown test would have trouble with - wind conditions, road surface variations, etc.
I think it is obvious from the charts that the task of writing a regulation would be very difficult.
So focusing on that one chart - RRC by tire size - it says that small tires (and here I am using "large" and "small" to mean load carrying capacity) have worse RRC than larger tires.
I ran a regression analysis on the 3 numbers in the tire size and developed a formula that best fits the data in the chart. Basically, the formula says that increasing any of the 3 number in the tire size helps RRC - for short: Bigger is Better.
But the percent change is small - a couple of percentage points for what will fit under a given fenderwell without rubbing.