The narrowest tires available for mtn bikes is 26 X 1.25 and they are not bad at all on the road, but a lot of the rims are too wide for them. I expect the sheldonbrown.com link above would let you know if you can get them for your rims, or if you would need to get new rims. With a lot of people owning mtn bikes and just lately starting to want to go places in the city, this question is going to pop up a lot.