Here's the test: Near Port Townsend, Washington, where Gear Guy Enterprises has a satellite office, we have county roads and state roads. The state roads are all asphalt, nice and smooth. The county roads are "chip seal"basically, sprayed with oil then covered with a layer of crushed rock. Passing cars eventually pound the rock into the oil, creating a semblance of a modern road, albeit a very rough one. I will go so far as to say that the difference between the asphalt and thechip seal is about that between a slick tire and a knobby, in terms of the"bumps."
On to our experiment: If I am going 18 mph on the smooth asphalt and then roll onto chip seal, my speed instantly drops to 16 mph. I'm also forced to drop a gear if I want to maintain pedaling cadencea 2-mph difference is roughly 11.5 percent.
My own estimate is that given that knobbies are wider than most road tires and run at lower air pressure, the actual friction difference is more in the 15-20 percent range.
So roll on, brother, you're right and the dealer is wrong.