[Blue Bomber Man]

From my understanding it works something like this:

RRC x weight = total energy loss.

so reduction of weight or RRC have similair effects, however RRC is easier to reduce provided that you can find reliable RRC values. I think the best tire on the market right now is actually a Bridgestone 14" tire that has a RRC of .0065, where as most tires have around an average of .0100

RRC is an independent variable of tire diameter, this is no direct relationship. However based on the averages of different sizes the study found that overall the large 15-16" wheels had lower RRC.

To answer your question: If a 12" and a 15" tire have the same RRC, then they will both yield the same fuel economy if there was no other effects at play (larger mass of the 15" tire, or the gearing disadvantage of the 12")

RRC values will dominate any effects of gearing or weight in general. In fact at lower speeds (probably sub 20mph) RRC will dominate wind resistance.

RRC is less significant at higher speeds compared to aerodynamics because drag works at the cube of speed.