The truth about rolling resistance:
Wide tires offer more control, more comfort and more fun. While this has become more widely accepted over the past few years Sport Scientist Peter Nilges, was not too interested, he was more concerned with speed.
The avid, German League road racer wanted to determine the best tire width and air pressure combination for fastest performance. He completed 300 test-rides with SRM cranks to establish the exact energy consumption for his thesis "Road Rolling Resistance". He tried three different tire types in three widths at 20 psi, 30 psi, 45 psi and 55 psi on road, dirt track and cross-country The results were clear.
Higher pressure was only faster on the road. Off-road rolling resistance was lower, the wider the tire and the lower the pressure. This was similarly true for dirt tracks, soft forest roads or cross-country and up to 40 Watts could be saved in extreme off-road conditions; poor acceleration caused by higher tire weight being generally compensated for. Explanation: A tire at low inflation pressure adapts better to uneven surfaces. It sinks into the ground less. Overall it suffers less retardation.
Narrow tires are only recommendable for lightweight riders, weight saving or extremely muddy trails.