I read this very interesting study the other day on tire rolling resistance. A common idea many people have that the higher the pressure and the narrower the tire the less rolling resistance you have. While this is true on the road any time you are off road (including gravel or hardpack) the less pressure you run and the wider your tire the less rolling resistance you have. In the study they show that a 2.1" tire at 57 PSI has 50 watts more rolling resistance than a 2.4" tire at 21 PSI with the same tread pattern. The argument that the wider and thereby heavier tire slows you down is true but only by 4.2 watts per 500 grams of weight during acceleration only. Tread pattern does make a difference as expected. Low profile/semi slick tires were proven to have much less rolling resistance than tall knobby tread patterns. So wide semi slick tires run at low pressures have by far the least rolling resistance with weight making a small but measurable difference. This was kinda what I thought all along but I was very suprised to see the wider tires having less rolling resistance. I've always been a 2.0-2.1 semi slick type of guy but maybe now I'll have to look into those 29x2.3 Karmas for next season.