"John Henderson" wrote
F Murtz wrote:

What about the fact that the skinny tyre is often run at over 100
PSI
and the fat tyre at 40 PSI

Run the wide tyre at its rated maximum instead of 40 PSI. It'll
be more comfortable, stable and faster than a narrow tyre run at
the same pressure. There are 2"/50mm tyres with 65, 70 and even
85 PSI ratings.

Thare's no doubt that your 40 vs 100 PSI comparison favours the
narrow tyre for rolling resistance.

Aerodynamics also favours the narrow tyre for lower drag, but not
at commuting speeds.

We've had this discussion before and there's no argument that the
rolling resistance of a wide tire and a narrow tire are the same when
both are at the same pressure. However, the narrow tire will
invariably have a higher recommended tire pressure, so when a wide and
a narrow tire are inflated to their recommended pressures, the narrow
tire will have a smaller contact patch and a lower rolling resistance.

The argument that a wide and a narrow tire have the same rolling
resistance is therefore spurious.

Theo