There have been a lot of anecdotal claims in the mountain bike world about rim width and rolling resistance. A new study funded by the Swiss Federal Institute of Sport should put the speculation to rest. As it turns out, rim width has a negligible effect on off-road speed.
Researchers compared three tubeless wheelsets: a baseline rim with a 25mm inner width and two rims with 30mm inner width. The first 30mm rim was paired to a tire with the same stiffness as the baseline and the second 30mm rim had the same tire pressure as the baseline. Then three riders conducted 75 rolling resistance tests a piece with each wheel over a cross country course.
All told the wider rim with the same tire stiffness decreased rolling resistance by 1.4% and the wider rim with the same tire pressure increased rolling resistance by .9%. The 1.4% improvement might not seem entirely trivial, but the corresponding effect on speed definitely was, 0% to .7% faster.