Q: My daughter had a flat tire recently, so I helped her put on the full-size spare. The next day I brought the flat to my neighborhood tire store and asked to have it repaired or replaced. It was too damaged to be repaired so we just replaced it. But here's my question. I've always heard that it is "better" to buy tires either four or two at a time, instead of just putting one new one on the car. Is that true? And, if it is, why is it true, and what makes it "better?" -Ted D., St. Paul
A: The demands on our tires change constantly as we accelerate, stop, take off and maneuver turns. Generally though, the front tires wear the same side to side and the rear tires wear the same side to side. If all four were new together and have been rotated (which requires that they all be the same size), the wear at all corners, and potential traction at all corners, will be about equal.
Comparable traction side-to-side means predictable handling on wet roads, dry roads and snowy roads. If you have a bald tire on one side and fresh tread on the other, the tread-deficient tire will slip first on wet or snowy roads. Technology like ABS and traction control will minimize the consequences. On dry roads, tread depth is less important than the tire's composition or age. Some rubber formulations grip harder; also, as motorcyclists know well, tires often harden over time and their traction declines.
So, one tire or two? If the other three tires have minimal mileage and you can buy the same size and kind, it's hard to see much harm in just buying one. If they have higher mileage, two new ones will keep traction consistent side-to-side. You mention your daughter's car having a full-size spare. If that spare is unused and you can buy a tire just like it, you can put one new one and the spare up front or at the back and move the blown tire's companion to spare duty. That approach gives you the same wear side-to-side while replacing only one tire.
The tricky situation is when you have a mini-spare and three of your tires are well into their useful life, say 1/2 used up or more. Let's look at the two approaches under this scenario. If you buy one, it's cheaper. You mount it and now have full tread on one side up front or at the rear and shallower tread on the other side. On dry pavement, this is not a big deal as far as traction goes. In the snow, other things being equal, the tire with the fresh tread will hold the road better and the other side may break free when pushed (your ability to notice will be minimized by the technologies described above, if present).
If you rotate your tires, you'll transfer this oddball tire around the car. When it comes time to replace the others, you'll have the inverse situation - better bad-weather traction at three corners and less at the fourth. Replacing just one tire means you'll confront the decision of whether to buy solo or in pairs again.
Replacing two is not inherently better in all cases; it's sensible when the companion tire is worn. In the rare event one tire fails early in its life, as from a cut or other damage, I wouldn't replace both it and its nearly-new companion. In that case, I would just buy one, assuming I can get the same kind, size and tread pattern. For activities that place uncommon demands on tires, like racing and four-wheeling, always replacing in pairs is a good rule. These situations test traction's limits constantly; having inconsistency in that situation is not sensible.