Originally Posted by Retro Grouch

Rolling resistance: What is it really? Look at a tire on any loaded vehicle. There will always be a flat spot at the bottom. That flat spot moves around the tire as the vehicle rolls down the road. Now take a naked tire and try to push a flat spot into one part of the tread. That will require some amount of energy. On a bicycle that energy ultimately has to come from the rider. If everything else is equal, a tire with a thin supple sidewall will have lower rolling resistance than a tire that has stiff sidewalls. A tire that has a thick anti-puncture layer will have greater resistance than one that has a thin tread layer.

Now look at the shape of the flat spot on the bottom of the tire. The size of the flat spot will be approximately equal to the load divided by the tire's internal air pressure. A wider tire will have a short contact patch. A skinny tire at the same air pressure will have a long narrow tire patch. Remember the energy required to form a flat spot in the tire? If everything else is equal, at equal air pressure a wider tire will have lower rolling resistance than a narrow one. That means you can run a wider tire at lower, more comfortable, air pressure and enjoy the same rolling resistance or you can pump up a narrow tire harder to achieve the same rolling resistance.