First, it is widely known within tire engineering circles that Treadwear, Traction (especially wet traction), and Rolling Resistance are the three legs of the technology triangle when it comes to tread compounds. Improvement in one property results in degradation in another.
I go into it a little deeper here:
Barry's Tire Tech
And RR is pretty much about tread compound: The amount and its properties.
Worn tires do indeed get better fuel economy than new ones - all other things being equal.
But tires designed with RR in mind - like Original Equipment (OE) tires - can have fairly low RR values compared to replacement market tires. But the tradeoff is traction and/or wear.
So, no, Frank, low RR tires do not always have thin treads - it's a bit more complex than that.
I was extremely disappointed in this report. While they may have tested some tires for RR, they did not test tires for traction and treadwear. I can understand that treadwear is expensive and highly variable - and that perhaps cost was a factor - but they didn't realistically deal with the marketing nature of the UTQG treadwear rating.
I was also disappointed in Marion Pottinger's stance within the committee. I sensed that there was a political agenda at work - that encouraging the concept that improvements in RR can come without sacrifices in traction and/or wear would result in more testing for those companies that do this type of testing - and guess what Marion Pottinger does? M'gineering is a testing company.
Bottomline: I found nothing of value out of this report and found much to be critical about. I think this report slowed the progress of tire technology rather than encouraged it.