Originally Posted by willtsmith_nwi
For DirtPedlarB, the casing of a bicycle tire is not rigid enough to hold a fixed height. If you take a tire and put it on a narrow rim, it will increase the diameter. If you put it on a wider rim, the tire will increase in width and decrease in height. And yes, lower air pressure will increase the size of the contact patch and lower the mean effective diameter of the tire.
So go ride your bike with the rear tire at 40psi and then go ride it at 20psi .. I guarantee it's harder to pedal at 20. But the 20psi tire will have a lower effective diameter so it should be easier according to this logic and what you said about friction not depending on area before.
I agree that it depends on the specific tire and ride whatever you feel comfortable with, but there is a penalty for big cushy tires.
weight doesn't matter too much, but the big tires deflect more so I'd say the width is more important than weight. The spacing of the knobs is the main thing I look at though. A tighter spacing will roll better, but may reduce traction. It's all a compromise when it comes to tires.