From Our Expert Technical Consultant:

Out-of-balance tires will cause a car to vibrate at certain speeds, usually between 50 and 70 mph. A tire is out of balance when one section of the tire is heavier than the others. One ounce of imbalance on a front tire is enough to cause a vibration in the steering wheel at about 60 mph. To balance a wheel, the technician will mount it on a balancing machine which spins the wheel to locate the heavier part. He will then compensate for the heavy part by attaching a lead weight on the opposite side. Many people are pleasantly surprised at how smooth their car drives after balancing all four wheels. Most high quality tires will hold their balance fairly well and go out of balance very gradually. If you notice a vibration that wasn't there the day before, it is possible that one of the lead balancing weights fell off. If you feel the vibration mostly in the steering wheel, the problem is most likely in a front wheel. If the vibration is mostly in the seat, the problem is in the rear.For those of you who are very sensitive about vibrations and your shop can't seem to get that last bit of vibration out, check to see if you have locking wheel lugs. Some locking lugs are about 1.5 ounces heavier than the other lug nuts which translates to about 1/2 ounce at the wheel rim. Try putting a 1/2 ounce weight opposite the locking lug and see if it helps.