Both tires should have nearly the same circumference. That is critical if the car has anti-lock brakes. Then all four tires must be the same. Even if your car has anti-lock brakes and the tires are mismatched, the computer will detect that, turn on the yellow ABS warning light, turn the system off, and you will still be able to drive like normal. The base brake system will not be affected.
What you might notice, if you care to make the observation, is the car might drift to one side of the road when you let go of the steering wheel. That is not only a result of different tires sizes, but also of different tire brands and models. They will have different rubber compound and develop different rolling characteristics. The car should drive fine, just be aware that there could be minor handling characteristics that you never noticed before.
Very often we have to tell people we can't align their car properly because they have two, three, or four different tire sizes. They were driving like that for a long time without complaint.
Keep in mind my recommendation to drive the car doesn't mean you should leave that mismatched tire on the car, but it isn't going to hurt for a few days. Braking traction and handling response in emergency maneuvers will be different but there's a good chance you won't even notice. Look at the mini spare tire that comes with most cars today. Now THERE'S a different tire size, but the circumference is still the same as the regular tires.
Saturday, June 2nd, 2012 AT 4:00 AM