[gone-ot]

...when a rubber knobby is bent over, it absorbs energy, but when it's released from being bent over, not all of the energy is returned as "springy-ness" in the rubber, but rather is wasted as heat.

...less rubber to absorbe energy = less energy stored, and thus less energy not being returned and ending up as wasted heat.

...some of the newer "tread" designs incorporate "deflection bumpers" in between the rubber block pieces to lower rubber-on-rubber contact which is purely "heat-generating."

...here's what Consumers Report says: "...As tires wear out, you’ll likely see improved fuel economy since the tires will have less rolling resistance as the tread (mass) is removed from the tire. Installing new tires, identical in brand and model as those removed, and you likely see a reduction in your fuel economy."