Originally Posted by somnatash

I also would say that in general fatter tires are not faster than slim ones. Rolling resistance and "air resistance" are important for speed. The faster the speed, the more air resistance gets important and the less important is rolling resistance. In road bikes (which are fast) the air resistance the the more important factor and of course slim tires have less air resistance than fat tires.

plus, the probably more important factor is, that fatter tires are (all else the same) heavier than slim tires. Weight slows you down. So these factors are reason for slim tires in road racing bikes.