Bicycle Mechanics - One slick tire on mountain bike. Should I replace the front or rear tire?

I have a mountain bike, and I'm thinking of replacing one of the tires with a slick tire like one on a road bike. I'm a bit cheap so I don't want to replace both, so I'll just do one.

Would there be a difference whether I replace the front tire, or rear tire? If so, which one would be better?

Replace both.

In an ideal world, both is the correct answer. You said you don't want to replace both because you are cheap and don't want to - this may very well be your proud way of saying you are broke and can't.

If it were me I would probably replace the front as a slick tire will give you more traction on pavement and you always want your best tire on the front. If you have an imbalance of traction and one tire lets go before the other pray it is the rear, because when the front tire lets go you are almost certainly going down, but you have a decent chance of recovering from a rear wheel slide.

However, the most effect would likely come from putting the road tire on the rear as most of your weight is over the rear tire and the resulting tire flex is what causes rolling resistance, and a road tire is generally made to have less rolling resistance than a mtb tire. Also you generally put more pressure in a road tire which decreases the amount of flex.

So my recommendation is to get a decently wide road tire (similar width to the mtb tires you have) so your bike is not tilted fore or aft and making it feel and/or handle weird.

And inflate both the road and the remaining mtb tire up to their max pressure for least rolling resistance.

Find a treaded tire and put it on the front. This one is affordable: http://www.performancebike.com/bikes/Product_10054_10551_1035343_-1_400904__400904

Unless you're looking to get a skinnier tire while you're at it, you can try sanding all the knobs off. It'll be reasonably fast with a power sander. Do note that puncture resistance will lessen, and it requires some skill. If you go too deep, you'll weaken the tire.

You can get slick MTB tires for ~$15. Check Nashbar, Performance, etc.. Too much for you?

Slick tire on the rear.

There's no automatic answer. It partly depends on your ultimate goal. I assuming that the bike is being converted to a pavement use bike.

If you want the most immediate benefit, put the slick in back because that's where most of the rolling resistance is generated.

But assuming that you have a long range plan to make this a road bike, and also assuming that the new slick tire will be narrower than the old tires, then you'd want it in front. That offers 2 benefits. The wider tire is the more loaded one, the tire you want to get rid of will wear out faster, getting you where you want to be sooner.

If you put the new tire in back, there's a good chance that it'll wear out and be replaced while you're still ridding the old one up front.

rear.

i have knobbies (MTB) and will replace the rear with a Schwalbe Marathon Plus (26 x 1.5"). When the front knobbie wears out I'll replace it as well.

start with the worn tires.

15 USD tires suck, but something good if you're changing from knobbies to slicks.

If they're the same width, I'd put the slick tire on the rear. I have this setup on my winter MTB (knobby front, slick rear) because when the ground is snow-covered the knobby front keeps me upright, but the slick rear keeps the bike a little quicker and a lot less noisy on dry pavement. Plus I can still hit the single-track with it if I want to.