My steering wheel vibrates on the motorway at certain speeds - what's wrong?

My car is in perfect condition but whenever I have the need to overtake or get out of trouble on a motorway the steering wheel starts to vibrate via the wheels when I hit 75 - 80mph. Below and above is fine. I have been to my local garage and a typical tyre garage, but no-one can fix the problem. What is wrong?

Asked on 13 March 2019 by James. V. Ellis

Answered by Honest John
It's probably caused by a disparity in the tyres. They must all be the same make, same tread pattern and within 3mm difference in wear.
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