Very few people buy only the same brand of tires when they buy just two. The tire size is important for the speedometer to read correctly. With 4-wheel-drive trucks the tires must all have nearly the same circumference, but that's most important only when actually in four-wheel-drive, which is not done on dry roads. The biggest factor is the tires must have the same circumference when you have 4-wheel anti-lock brakes. If the tires aren't exactly the same size, one will rotate faster than the others. That will cause false activation of the ABS system because the computer will think one wheel is slowing down faster than the others.
You can buy two new tires this week, and two of the same brand and model next week, and they can be significantly different, so you can be sure two pairs of tires from different manufacturers are likely to be different. Except for a couple of vehicles that have full-time all-wheel-drive, those slight differences in size shouldn't cause a problem.
Tuesday, November 20th, 2012 AT 2:48 AM