For six months I just pottered about in my car with no problems, then two months ago, during a 250 mile journey, I could feel the steering wheel AND the seat vibrating at 77, oops I mean 67.  At the time my back did not feel uncomfortable but the next day I had a serious back ache.   I had the front wheels rebalanced and the tyre shop said that the seat vibrations indicated that the rear tyres could be shaped like fifty pence pieces, although they felt the OD of the tyres and could not feel any