"Wide tires do not roll slower at lower pressures. In fact, testing indicated that a wide tire at lower pressures rolls faster than a narrow tire at high pressures, if all other factors remain the same. Even narrow tires can be ridden at comfortable pressures with only very small concessions to performance.
Perhaps the most important result of the test is that tire pressure does not significantly affect rolling resistance. Wide tires in particular do not need high pressures to roll fast. But because many current wide tires are designed to handle high pressure, they have strong casings that lack suppleness. This results in higher rolling resistance than necessary."
- Road Bike Rider Newsletter (2006), based on Vol 5 No. 1 issue from Bicycle Quarterly http://www.bicyclequarterly.com. The test included nine 700C tires, seven 650B tires and two tubulars. The protocol and results were reviewed by industry experts.
Courtesy : Velo Chimp