How to match dual tires when replacing one new tire.

Had a question on my video on matching Dual tires.
"I get the importance of size-matching, but how are you going to get an adequate & accurate measurement of the remaining dual while it's still on the truck or coach? It'd be expanded under pressure, somewhat distorted where it presses against the ground, and would have some wear on it. Is one supposed to compare a tire under those conditions to a new, unworn, un-inflated tire & expect them to be within 3/4?"

Well I can understand the bit of confusion as "Dual Matching" can be a challenge when trying to replace one tire on an RV. Lets work through the process.
First off we need to be sure we are doing the correct thing in replacing just one tire from a dual pair. Normally people have one dual tire fail due to some form of damage and subsequent air loss. In the past, before TPMS, the driver would continue down the road till someone waved him over pointing out thr tire failure. This meant that for some unknown number of miles the remaining dual had been operated at 100% overload. The general guideline would be that you need to consider the remaining tire to have had its structure damaged and it should be replaced. There are many documented cases where only one tire was replaced and the damaged tire fails a few miles to a few weeks later. At that point the driver acts surprised there was a failure.

Today we have TPMS available so that in the case of a puncture or other slow leak the driver may get adequate warning so he can stop before the tire with low inflation suffers a catastrophic blowout because it was run with 20% or greater air loss. In this case the fully inflated tire may not have been overloaded for more than a mile or two as the RV moved to a safe place to pull over, so after internal and external inspection it may be safely retained and put back in service.

Now the RV owner is confronted with the challenge of how to properly match tires for dual application. The 3/4" circumference is the correct maximum difference for tires and tire growth and wear can make it a challenge to meet the goal.

When measuring a tire for dual matching it needs to be off the vehicle and fully inflated. If the tire is brand new you can expect even radial tires to see some size increase after a couple hundred miles so it is difficult to match new and used and get correct numbers.

The solution if you are replacing just one tire would be to confirm your front tires meet the 3/4" difference and then place a new tire on one front and the old tire on the other front position.

Meeting the 3/4" max difference is critical as ignoring this figure can result is a failure down the road even if the tire has been properly inflated.