Tires: squirley replacement tires, factory rims, replacement tires

Question
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    The tire pressure on the door placard is 45lbs front and 80lbs rear on 16x6.5in rims for 8800lb gvw. Chrysler recommends 45lbs front and 40lbs rear for light load (empty). I have always inflated the tires to 45lbs front and 50 lbs rear while running the truck empty and have had no problems. When I haul my fifth wheel I inflate to 50lbs front and 80lbs rear. I get even tire wear on the front and rear and experience no problems and is rock solid with no pull to either side of the road. The truck is as perfect as can be with my old tires.
    The original factory rims were steel 16x6.5 inch chrome and that is what is still on the truck. I try to stay stock with everything thinking the manufacturer knows what is best.
    When I put the new tires on, this is what happens:     
    Say you are driving down the road at 40mph or more and turn the wheel to the right, then left, then straight, like you are going around an object in the road. The truck wants to squirm back and forth for 3 or more times. By squirm I mean the steering wheel is straight and the truck is going left and right. The faster you go the worse it gets. If you stand beside the truck at the rear wheels and grab the top of the bed and shake it back and forth, the truck moves side to side several times. You can actually see the sidewall of the tire move side to side on front and rear. The tread area is stationary and the truck is moving side to side. Say you are exiting the freeway, you turn the steering wheel to get on the offramp. It feels as if the truck is going to spin out. All of this does not happen with the old tires. It does happen with new tires exactly the same size as my old ones.
    I dont know how else to describe this condition. I can understand this condition if the tires were severely underinflated but this is not the case. I check the pressure myself as I don't trust the installers to do it right. I even retorque the lug nuts to rule that out.
    I have put Bridgestone, Michelin, and Goodyear on the truck in the same size and load range as the old tires. The old tires are Goodyear Wrangler RTS 245 75 R16 E. The new Goodyears, which were exactly the same as the old ones were the worst. They felt squirley or squirmy at 25mph. At 50mph it felt as if you would lose control of the vehicle. The Michelins felt the best but still they moved around too much. If you take the new tires off and put the old tires back on, the problem goes away. I would think that would rule out worn front end parts and anything mechanical but I still had the vehicle checked out.
    I hope this helps diagnose the problem.
         KEN.   




































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Answer
Ken,

Thanks for more detail.

I don't think we've completely eliminated alignment as a contributor, because a tracking problem can cause this and tracking is something that can be masked by some particular tire properties.  Besides, it wouldn't be the first time a vehicle manufacturer "fixed" a problem by altering the tires they spec'd - with the result that almost every other tire was unstable.

I'm guessing you've encountered this problem only using the "empty" condition with the "empty" pressures - and you've never tried hauling the 5th wheel with these other tires.

Just for the sake of discussion, I'm tempted to tell you to run 10 psi higher in the rear than what you've been running.  And, yes, that includes the fully loaded condition where you use 80 psi in the rears (I'm recommending trying 90 psi), even though the sidewall of the tire says max 80 psi cold.


But other than that, I can't imagine why sets of tires would behave so differently, unless the vehicle has a peculiar situation.

But try the increased pressure.  I'm not sure that will help, but it seems intuitively right.