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I think Tire Rack has this right. It's about energy consumption.
If car A consumes 50 mpg and only 2 mpg are for tire rolling resistance, then the car changes tires (and let's pick an extreme example) now gets 4 mpg for RR, then the net is now 52 mpg (52/50 = 4% improvement)
Car B consumes 25 mpg of which 2 mpg is tire RR then a change to 4 mpg for RR results in a 27 mpg (27/25 = 8% improvement)
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