Mountain Biking - Slicks VS Knobby tires

I was in a local bike shop looking at some slicks for a century in December. A couple of the hourlies there say going from an average set of knobby tires to a set of 150psi slicks will add 3 to 4 miles per hour to my speed. Sounds a little to good to be true. Anyone out there had experience doing a century with similar equipment. ie. A moutain bike with slicks. I am doing about 60 miles a week at 14.7 to 15.7mph now.
Thanks

well there's two different things going on here. You have road tire slicks and mountain bike tire slicks. IF you get the road slicks it will require you to have different sized rims and depend on what kind of break set up you have they probably wont work but that will add extra MPH to you overall total.
Mtb slicks on an mtb rim will also add extra MPH to your overall but not as much as the road/road set up.
I did an MS 150 (houston to austin 182 miles) two years ago on a mtb with slicks/mtb rim and it was coush.
So in summation:

Yes it will add extra MPH to your totall but how much will depend on your setup.
t

Going from knobblies to slicks is the single biggest improvement you can make for riding on the road.
Knobbles are like small bumps that suck your energy and turn it into noise.
Slicks are narrower, so can take a higher pressure which reduces rolling resistance.

The width thing is a bit more complex. At the same pressure a wider slick has a lower rolling resistance, BUT narrower tyres can generally take higher pressures.
For distance riding, a slick of 1.25 to 1.5", pumped to its max will be effective.

My touring partner had fat knobblies. When one burst, I convinced her to change to slicks and she was amazed at the difference, and was still able to use them on trails.

I will tell you that I averaged 2MPH faster from one day to the next. Going from knobbies to slicks on my morning training rides. There was no question on how much nicer it was to be on some nice tires. I could just coast down most hills that I was having to pedal down to maintain my speed.

Now if they are saying you could gain 3-4mph going from a mtb to a road bike, I would almost have to agree. That is about what happened to me when I switched. I picked up almost 2mph on my rides going from my MTB (with slicks) to my current road bike (Jamis Nova with slicks).

Going to slicks added a couple of km to my average speed and increased my endurance.

Originally posted by Funnyman
I was in a local bike shop looking at some slicks for a century in December. A couple of the hourlies there say going from an average set of knobby tires to a set of 150psi slicks will add 3 to 4 miles per hour to my speed. Sounds a little to good to be true. Anyone out there had experience doing a century with similar equipment. ie. A moutain bike with slicks. I am doing about 60 miles a week at 14.7 to 15.7mph now.
Thanks

Going to narrow slicks from fairly wide knobbies will make a considerable difference, though I think 3-4 mph is a bit optimistic.

I don't know of any 26" slicks which are suited to such high pressures. 100 p.s.i. is about the limit for the 1.0 and 1.25 x 26" slicks I have seen/used (I use the Specialized Fat Boys, 26 x 1.25").

If your rims are fairly wide, you will probably be best served with a 1.5" tire. If you can use 1.25, this will be even better for your avg speed though with the greater pressure necessary with narrower tires the ride comfort will decrease.

The best reason to use slicks is for increased traction, something you will especially appreciate going around corners at high speed.