Tires: tire upgrade, goodrich tires, tire fitments

Question
I recently went to COSTCO to purchase 4 tires for my 2000 Chevy silverado. Its a 2500 ext cab. the Goodrich tires they installed last year are starting to loose chunks of tread and wear down the center of both rear tires and all 4 tires have a bevel in the sidewall. I don't tow anything and I don't go off road. The only off road it see's is maybe when I go surf fishing. I want to upgrade my tires to Michelins. I want them to swap out the old for LT265 75R 16s
and they tell me that they can't do it because its their policy to only swap what the mfg. says. They say I now need a letter from a dealer stating thats its ok to install these tires. HELP!!!! Is this true?
Thanks for your time  in this.

Answer
Bob,

I hope you are not asking if it is true that Costco's policy is to only replace tires with the original size.  I think you already know the answer to that question.

Instead I am hoping you are asking if there is a problem with changing tire size - and the answer is: "Yes, there is."

1)  Anytime someone does something "non-standard" - and something bad happens - the courts tend to think that someone in a position of authority should have prevented that from happening.  As a result, companies have policies that their employees will follow the standard rules - and one of those rules is that tires have a vehicle placard and the tire size listed on the placard is the proper size for that vehicle and using a size not listed on the placard MIGHT cause a problem, so don't do that!

Remember, the folks that bust tires are not rocket scientists, so not only can't they be trusted to be knowledgeable in the many aspects of tire fitments, they can only be trusted to partially follow the rules.

2)  So what are the possible problems of using a size different that what is on the vehicle placard:

 a)  The tire might rub.  This could, in extreme cases, cause an accident, but at the very least it is annoying and a potential source of a complaint.  The only way to know for sure if a larger tire is going to fit is to measure the area inside the fenderwells, including all the edges of the fenders and the frame rail locations and the way the suspension travels up and down, and how the steering affects the location of the tire relative to all that movement - and this not only takes time, but requires a certain amount of expertise.  Needless to say, some tire dealers have this expertise, and think it gives them a competitive edge - so they do not share that information.

  b)  The tire might be too large for the width of the rim.  This could cause a wear problem or some vehicle tracking issues.  The only way to know what the width of the rim is - and what the requirements for a given tire size are - is to have reference books - which kind of gets us back to the expertise issue again.

 c)  Using a tire of a different size changes the location of the contact patch relative the suspension.  The Physics of vehicle suspensions can be quite complex.   A simple thing as using a larger diameter can change the the way a suspension reacts and that could potentially cause a handling issue.

I could go on, but let me address the issues you raised, and didn't ask about:

1)  The rear tires are wearing in the center:  Drive tires tend to do that - the fronts tend to wear on the shoulders.  That's why it is important to rotate your tires every 5K to 8K miles.

2)  All 4 tires have a bevel in the sidewall:  Not sure about this, but is it possible that you are using an inflation pressure that is too low?  

On every vehicle sold in the US, there is a sticker – commonly called the tire 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.

I suspect that the pressure listed on the placard will cause the vehicle to get stuck in the sand when you go surf fishing - and so using a lower pressure would prevent that, but that means the tire has to be reinflated as soon as it hits the street.

That could be both the source of the beveling and the chunking.

I am suggesting that the source of your problems has nothing to do with tire size.  Plus some versions of your truck came with 6.5" rims and an LT265/75R16 requires a minimum of a 7" rim.  I think you need to talk to someone knowledgeable about tires - and you won't find him at a big box store.  You'll have to find a local retailer who is dedicated to tires - and only tires and their related components.  That guy should be able to help you select a tire appropriate for your usage.