Need your experience

classic 31 wrote:
With all that vibration and you say you can't tell if it is front or back I would have my doubts if it was from the tires at all maybe from the engine running and misfiring .if the tires were that badly out of round you would of felt something long before 40 mph.maybe the engine has some sort of high speed miss .and it is shaking the whole unit.

Not necessarily. Different amounts of out of round tires, present different effects, at different speeds. I've had my share of out of round tires and, based on all kinds of factors, size of tires, type of suspension, attempt at balancing, awareness of driver and more, the effects will come in at different speeds. A heavier tire and, heavier rig quite possibly will suppress the effects of an out of round tire until it can't. Then the centrifugal effects take over, as the tire spins at a higher rate of speed.

And, a potential engine misfire, if it's constant, will have way more of an effect at much lower speeds when the vehicle is just starting to accelerate. That's when the engine is laboring to get the massive weight moving. And the misfiring cylinder would be more easily felt. But, as the speed increases, the individual firing effects of all the normally operating cylinders, will mask the one cylinder that is misfiring due to the fact that you'd not miss the missing power from an individual cylinder other than a bit loss of power.
Scott