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Wednesday, January 16, 2013

Can I mix Load Range ?

There was a question about mixing tires of the same size & design on an axle if the Load Range is different.

First we need to all understand what is meant by "Load Range". The official definition as published in the Tire & Rim Association yearbook says "Load Range with a letter (A, B, C, etc), Standard Load, Light Load and Extra Load are used to identify a given size tire with its load and inflation limits when used in a specific type of service, as defined in the heading of TRA tables".

Basically "Load Range"identifies the max inflation for that specific size and type tire and the load capacity of the tire when set to that pressure. many think of it as Ply Rating 2, 4, 6, etc. I previously covered Load Range and Ply Rating in THIS post.

But what do you do if you have to replace one tire on an axle because of some failure like a puncture, impact or even a "Blowout" and can find a matching tire of the same size and type but of a different Load Range?

If you are replacing the failed or damaged tire with a higher Load Range you can run it as if it was of the lower Load Range.

Example 1. You have an ST225/75R15 LR-C and can only get a LR-D.  Your Placard indicates you should run 50 psi cold. Since you know you should always run all tires on any given axle at the same pressure your only option is to inflate the LR-D replacement to 50 psi. Later on you may want to increase your RV load capacity so you may want to get another LR-D and then inflate both tires to 65 psi which will increase your load capacity of the tires by over 300#
 BUT you need to confirm the wheel is rated for 2540# and 65 psi. You will need to contact the wheel manufacturer to confirm the wheel is so rated. I am not sure if i would accept what either the RV dealer or even the RV manufacturer says as they might not have all the engineering data or have other reasons to state a lower capacity.
You will also need to confirm you are not overloading the axle or springs or frame by increasing the load.

There is another reason to increase the inflation but not increase the actual load beyond the original max of 2150#. By running the tire at the higher inflation you are giving yourself a larger "Reserve Load". Some consider this like a safety margin. By running the higher inflation at the lower load you will also decrease the internal structural loads the tire has to tolerate and this should give better durability.

Example 2. Your RV placard indicates you should br running 255/70R22.5 LR-H but you can only find LR-G. In this case you are really out of luck unless you know for certain that you have less than 5205# load and not something any higher. The LR-H tire is rated to carry 5510# at 120 psi while the LR-G is only rated for 5205# at 110psi. This may not sound like much but you should not run your tires in overload as you will do structural damage to the tire which could lead to a reduction in life and even a tire failure under some conditions.

If you are running the highest Load Range for your size you may want to consider carrying a spare tire and let the road service mount your old used spare to be used to get you off the highway. This way you will not be spending hundreds of dollars for a tire you can only use for a few miles.


1 comment:

  1. I have a 97 international 4700 utility body loaded with tools is it OK to have g tires or right rear and h tires on left rear same size different load range

    ReplyDelete

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