re: Q for old car people and mechanics here (followup from Ask a Mechanic)

Flat spots usually give you a thumping sensation at all speeds and usually makes noise.
Just did a quick internet search. This is from aa1car. Com:

quote:

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Symptoms of Tire Imbalance An out-of-balance tire and wheel will typically create a vibration or shake that becomes progressively worse as the vehicle speed increases. The speed at which the vibration first becomes apparent will vary depending on the size and weight of the tires and wheels, the size and weight of the vehicle, the sensitivity of the steering and suspension and the amount of imbalance, but usually starts at 40 to 50 mph and increases in intensity as the speed goes up.

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A rotation can't hurt an it's free.