[tasdrouille]

RRC is a tire's property that is important only to few people. You then don't have to test all tires, just the tires someone who cares about RRC would like to buy. Probably even fewer people care about the RRC of a pzero nero than there are ecomodders posting here, but if a company makes a tire with "eco" or "fuel" in its name suggesting that it's a tire with a low RRC, I woud like to know that value. Most manufacturers have what, 1 or 2 model marketed as fuel efficient? That narrows the field quite a lot.

Regarding correlation between RRC and size. For a given model, take a reference size and compare the rrc with other sizes so each size is a percentage of the reference size. You'll get a different percentage for every size and the correlation will not be linear and, like you stated, not completely significant. However, what would be interesting to do is perform the same test on a couple other models and see how the coefficients (percentages vs ref size) for a given size correlates between models. Say 195/50R15 is 1.1 of a 195/65R15 for tire model A, is 195/50R15 also 1.1 of a 195/65R15 for tire model B?