Join Date: Jun 2004
Location: Northern Nevada
from full-on knobbies to road tires makes a HUGE difference. I did my first few centuries on a Bridgestone mountain bike with road rubber, because that was all I had at the time, and it worked fine.
From one road tire to another, the difference would be smaller, and would depend on what you took off and what you put on. I haven't used completely slick tires in several years, but as I recall, there wasn't a big improvement over "nearly slicks." But if your present tires have heavy tread or are rated for low pressures, and you move to something with less tread and higher pressures (around 75psi or above), you'll feel it.
FWIW, I've been happiest with tires about 1.4-1.5 inches at 80-90psi. Skinnier or harder tires sacrifice comfort and control with no real improvement in speed.