Originally Posted by Don Cook

A larger tyre (25c vs. 23c) has less rolling resistence at the same air pressure than an identical but narrower tyre. This is because the larger tyre will have less sidewall deflection as compared to the smaller tyre when they are identically loaded. Independent testing of rolling resistence factors for bicycle wheels, pretty much discount the contact between the tyre and the road surface as contributing much to rolling resistence. The sidewall fexing as the wheel rotates and the wheel hub bearings contribute most to the resistence.