Tires: 1996 Dodge Load Rating., proper inflation pressure, tire failures

Question
QUESTION: I have a 1996 Dodge Ext. Cab 2WD 5.9L truck that i have recently bought size 245/70-16 tires to keep the speedo and ratios in check. the load rating is 2149 per tire. and the construction is the same as the current tire. 6 ply tread, 2 ply sidewall.

I took these tires to get installed and they would not do it because of "safety" concerns with the tire blowing out.

The current tire is a 245/75-16 with a load rating of about 3000

i have heard that i can and cannot run my new tire. which one is it?!?!?!

any help would be greatly appreciated.

thanks in advance

ANSWER: Brandon,

Correct me if I am wrong, but doesn't the placard on your truck call for LT245/75R16's inflated to....well, there's quite a variety of models in the book I'm referencing, so i can't tell, except it's at least 45 psi and probably 55 to 80 psi.

The vehicle placard is usually located on a doorpost or in the glovebox, and lists the original tire size and the proper inflation pressure for that size.

So take a look at the P245/70R16's you purchased.  Don't they say the max pressure is 44 psi?

Obviously there is a problem.  So you need to return the tires and obtain tires that are equivalent to what came originally.  BTW the letters are important - including the Load Range.

OK, I've told you what to do, now I'll tell you why.

The vehicle engineers that designed your truck spent a lot of time selecting tires that were capable of handling the load that your truck is capable of carrying.  I had a professor who use to say "Overdesign / underutilize".  In this case, the vehicle engineer is following decades of experience that tells him that if he selects a tire that barely carries the load, then lots of folks are going to experience problems with tire failures - something that he obligated to avoid.

Let's face it - a tire that the maximum load carrying capacity is 2094# at 35 psi is much less save than a tire that is capable of 3042# at 80 psi - even if the specified pressure is less than 80 psi.

So it is not surprising that the shop you took these tires to refused to mount them.  They are looking out for your safety - not to mention the legal liability they would incur by not informing you of this.

---------- FOLLOW-UP ----------

QUESTION: the placard calls for a 225/75-16 XL
i have found sources that indicate the stock tire was a max load of 1900

the tires currently on the truck are the 2nd set of tires purchased and are 245/75-16 and are supposed to be aired to 80 psi.

i dont plan on towing anymore with this truck so thats not an issue.

but isnt a wider tire capable of sustaining more load?
the NEW tires are 245/70-16 and are 2149 at about 40-45 psi.

so shouldnt this work?

thanks again for your help.

Answer
Brandon,

Good info.  But I want to point out that you left the "P" off the tire size and this might be important, as you can see when we start talking about LT tires.

OK, a P225/75R16 XL (Extra Load) has a load carrying capacity of 2094 pounds at 41 psi.

Well, the P245/70R16's also have a 2094 # load carrying capacity, but that occurs at 35 psi.  So this is OK, and the tire shop was wrong.  They of course looked at the current tires on your vehicle, which is not the right way to do this.

However, you've said the load carrying capacity of the tires in question is 2149 # and that tells me they are European based and the tire size doesn'y have "P".  BTW the pressure where this occurs is 36 (2.5 bar) psi, even though the sidewall will say 44 psi maximum.  You should notice that on the sidewall the way this is worded does not connect the maximum load with the maximum pressure.

Bottomline:  These tires are OK to use and the pressure should be 35 psi.

Wider tires?  No, wider tires may be just wider.  What is important is the load carrying capacity.

It turns out that if done correctly, you can get tires that are wider and have the same diameter and the same load carrying capacity.  Needless to say the relationship between physical dimensions and load carrying capacity is pretty complex - which is why this stuff is published in the form of tables.

Words of Wisdom:  

1)  The vehicle placard is the source for tire size and inflation pressure for that size.  

2)  If you change tire sizes, the pressure has to be recalculated.

3)  If your vehicle came with P metric tires (or metric tires), then you should not use LT metric tires.  If your vehicle came with LT metric tires, you should use LT metric tires.