Tires: Toyota Matrix Steering Wheel vibration, wheel vibration, wheel alignment

Question
What is the meaning of "out of round tires" ? And for this model how often should I do a tire rotation ?
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Followup To

Question -
I drive a 2003 Toyota Matrix, wagon shaped. I replaced the front two tires with new ones and the rear two tires are older. The rear tires still have about 10,000 miles left as per the tire shop though the threads are not as deep as the front ones. There is a jittering vibration on the floor panel at 58 m/p/h and 70 m/p/h and the vibration is intense which I can feel on my whole left leg. Its has an automatic transmission so my left foot rests on the floor rest plastic pad. Sometimes the vibration is also felt on the steering wheel. All the wheels have been balanced twice in the past two days and a 4 wheel alignment was done. The vehicle has about 60,000 miles on it. The vehicle has a 100,000 miles warranty but I do not want to take it in to the dealer unless its absolutely necessary.
What do you think the problem could be ?


Answer -
In all likelihood, what you have one or more out of round tires.  Since most of the vibration is in the floor, it's most likely that it is the rear tires.  Since it is possible to wear a pattern into a tire that can cause a vibration - and your vehicle is noted for this - neither the vehicle dealer nor the tire dealer are going to handle this under warranty, particularly since the tires need to be rotated regularly for any warranty to be applicable.

The only fix is to replace the tires.


Answer
"Out of round" means....well.....OK, I'm going to give you the simple version and as you discover more about "out of round" tires, you'll see that it isn't easy to be accurate.

We think of tires as being circles - and they are.  but like everything in this world, they aren't perfectly round.  They have a little bit of variation around the circumference.  If the conditions are severe enough, you can feel this variation as a vibration.

There are a number of different reasons why tires might be "out of round" and one of them is a misalignment that wears a pattern into a tire.  Another one is when a vehicle sits in one place for months and months and the tire gets deformed and forms a "flat spot" right where it contacted the road surface.

Tire rotation is determined by the vehicle manufacturer, but generaly it is in the 5,000 to 8,000 mile range.