It is common for a front tire to outlast a rear tire by as much as three to one. Rear tires have more weight on them, and also have to deal with drive forces.

This disparity in tread life is exacerbated in the case cyclists who rely on their rear brake (you shouldn't! See my article on Braking and Turning.)

Well-meaning cyclists, even some mechanics who don't know any better, sometimes try to deal with this by swapping tires, putting the less worn front tire on the back wheel, and moving the worn-but-usable rear tire to the front. The idea is to equalize the wear on the two tires, but this is a serious mistake, don't do it!

The only time tire rotation is appropriate on a bicycle is when you are replacing the rear tire. If you feel like taking the trouble, and use the same type of tire front and rear, you should move the front tire to the rear wheel, and install the new tire in front.

The reason for this is that the front tire is much more critical for safety than the rear, so you should have the more reliable tire on the front.

If you have a blowout, if it is on the rear tire, you have a very good chance of bringing the bike to a controlled stop. If your front tire blows, you can lose steering control, and a crash is a real possibility.