Tires: tire size, rim diameter, dualie

Question
I wanted to know if a tire - 70r19.5 - was replaced on the outside back dualie that has all the other tires on the vehicle sized r19.5, will this affect the performance of the vehicle, or cause damage?  In other words, is it likely that the other side, (the back right side) could be affected negatively by this tire replacement?  Is this normal practice by tire companies to replace tires with different sizes, such as the example of what was just mentioned?

Answer
Lana,

First, tire sizing consists of a number of different configurations, and the most popular has the form: XXX/YYRZZ, where:

XXX = the width of the tire - and typically the number will be slightly over 200
YY = the aspect ratio - and the number will be between 50 and 85
ZZ = the rim diameter - and for trucks, the number will be as low as 16 and as high as 25.

So a 70R19.5 only gives me 2 of the 3 numbers, so I can't tell what size that is.

And an R19.5 is only the last part.

But it is possible that you have tires of the form  XXRZZ, where

XX = the width of the tire, and encoded in the way it is written is the aspect ratio
ZZ = the rim diameter as above.

But where I think you are headed is the general rule that all 4 dualled tires have to be the same size / brand / make / and state of wear.  And it sounds like this is NOT what you have.

If this is true, then YES - emphatically, this is a bad situation.  The tires need to be the same so they carry the load evenly.  If they don't, then one of the tires could be overloaded and fail - and that causes all kinds of bad things - including loss of control and personal injury (or worse!).

If this is true, then this needs fixed NOW!!