Hopefully I'm not beating a dead horse here. I can't seem to find the answer in my search of the site.

How much difference is typical between Hoosier's or TT tires compared to 20" knobbies? Don't know if it matters much, but I'm just wondering because we have always dyno'd our stuff on the same set of 20x11x10 knobbies. We make just over 45hp. and 30ft.lbs with these tires. There are some people we hang out with that make 4+hp. and 4+ft.lbs. more than we do, but they don't seem to have any more power if we're going at it side by side. We're pretty even.

Can there really be that much difference between knobbies and "dyno" TT-style tires?

yes knobbies take away hp on a dyno..

Yes, tires do make a difference in overall HP being read. Going from 20" knobbies to 20" completely slick knobbies yield roughly 2-2.5 HP on my dyno.

Also, tire size and gearing will have a slight effect on an inertia style dyno as where there wont be a difference on an eddy current dyno. Best thing to do is use the dyno for a tuning tool, and dont worry about number output. Dyno with the same type tires you always dyno with, unless you are going for a new baseline. If you are going for a new baseline, start with some slick tires or flat track style tires....doing this HAS to yield less interference or fake loss due to resistance.