Tires: Wrong tire load causing problems?, chevrolet silverado, stock rims

Question
I have a 2000 Silverado 2500. It comes factory with 255 tires. I bought some used tires that are a size bigger (265) a/t and are load E. I didn't realize they were load E until like 3 months later. They rode fine for about 6 months and now when I get to about 60mph+ my truck vibrates like a jack hammer. Also I'm running about 40PSI instead of the recommended 80PSI because my valvestems wont hold 80. Anything I can do other than buy new tires?!?

Answer
Chase,

On every vehicle sold in the US, there is a sticker – commonly called the vehicle placard - that lists the original tire size and the proper pressure for that size.  The placard is usually located on a doorpost or in the glove box – but sometimes it is located in the trunk or on the fuel filler door.

Please look for the placard and record the tire size (including the letters) and the pressure.

I have a book called Tire Guides, that lists the placard information form most current vehicles.  Unfortunately, pickup trucks come in so many combinations and with so many different tire size, it's hard to tell you what the placard is likely to say without more information about the exact model of Silverado you have.  That makes it very important that you find the placard.

BUT

I can tell you that a 2000 Chevrolet Silverado never came with 255's.  It's likely that the placard is will say the tire size is LT245/75R16 on 6.5" rims - and the minimum inflation pressure I have listed is 45 psi and many of the models show 65 and 80 psi.

So to start with, you should get valve stems that are rated for 80 psi.

Second, you should be aware that unless you changed from the stock rims, the 265's require a minimum of a 7" rim.

So I think you have 2 problems - not enough inflation pressure and the rim is too narrow.  This probably caused the tire to wear unevenly around the circumference - something we call irregular wear.

But it is also possible that because you did not have enough pressure in the tires, the tire is separating - and this is a safety issue and must be addressed NOW!!  

Drive slowly (less than 35 mph) to a tire shop and have them inspect the tires for bulges and unusual wear patterns.  A separation will eventually cause an 8" long worn spot in one of the shoulders - right on top of the separation. Do this right away, because as the separation progresses, eventually the tread will come off and damage the fenderwell - or worse!!

I can not emphasize this enough.  Do not postpone this.  Fix this now!