Originally Posted by
CommuterRun
The skinnier the tire, the smaller the footprint it will have and consequently, less rolling resistance.
However, in my experience for commuting purposes the differences in tire width are inconsequential unless less than perfectly clean, smooth asphalt is encountered. Then wider will give you an edge.
If you are riding on rougher roads, large tires can be faster by giving you some shocks with the extra air.
I'd also disagree with filling up your tires to the max in these situations. In fact on road tires (which I know is not what you are riding), I pretty much never go over 110 psi, usually 100. 19mm road tires pumped up to 150 psi may feel fast, but the hard road vibration you'll get with this setup will slow you down in the end. That is one of the reasons mountain bike and cyclocross racers run tubular tires. So they can get the softer feel and additional traction tubulars can offer.
So, what I'm saying is if you ride rough roads or light trails, wider and lower pressure is going to be more comfortable and in the end faster.