Toad + Tire Monitor + How far over/under parameters?

Gsmitty77 wrote:
We had the Smart Tire II system, on 6 of our 8 RV tires. (Always made me LOL, that the factory installed Tire Monitor had capacity for 6 tires only:)!)
..........
TIA, and best to all,
Smitty

I had the same SMARTIRE 6 wheel system on a 8 wheel MH.
One day, while i was stopped at a rest stop for lunch and using my IR gun checking on tire temperature, i found it was running really hot. Turns out my brake calipers were over lubed and hence dragging tag brakes. If i had a 8 wheel TPMS i would have caught this before, from inside the MH

Recently when i changed my tires, i took the SMARTIRE TPMS out and out TST for all 8+4 tires.

I do have to admit that SMARTIRE was/is a superior product over anything i have experienced/seen. The batteries lasted over 13 yrs and are still working. But i wasn't sure it will last another 6-7 yrs until my next tire change so i had to take this out.

Getting back to your question:-
what matters is the pressure differential.
This is also how SMARTIRE does it.
So you may want their read their manual.

Recommended cold pressure is based on the load + 5% (safety margin)
Why because, chances are both sides of RV is not equally loaded.
I pick the highest pressure i need and add 5%-ish to it.

For my MY,

I run 95psi on drive axle tires (11R/22.5)(had 295/80/22.5 before)
I set cold at 90psi (must air up in this situation) and hot at 125psi.
I haven't seen drive tires exceed 125psi under load.
So pressure differential under load is 15psi

I run 105psi on steer axle (315/80/22.5)(had 295/80/22.5 before)
I set cold at 100psi(must air up in this situation) and hot at 135psi.
I haven't seen drive tires exceed 125psi under load.
So pressure differential under load is 20psi

I run 75psi on tag axle (11R/22.5)(had 295/80/22.5 before)
I set cold at 70psi(must air up in this situation) and hot at 100psi.
I haven't seen drive tires exceed 85psi under load.
So pressure differential under load is 10psi.

Rule of thumb is, every 10F rise in temp leads to 1psi pressure increase. So if your cold pressure was (say) 100psi at 70F (morning) and now you are running under load at 120F outside temperature, just for the cold pressure it is 5psi change. Add to that the heat factor, say another 20 to 30psi higher, your net tire pressure is 135psi which is still below the danger mark of 158psi

The pressure and temperature increase should taper off with time
The first 30min it will rise and then should be nearly steady

Having more air in tire will lead to less differential change in pressure & temperature (Boyle's law : PV=nRT or P/T = nr/V). But you may (will) get a harsher ride.

pplus 20%, under 10%

The amount the pressure goes up is not that important assuming it was not under inflated to begin with. 36 for the toad is probably on the high side so even if pairs of tires go up the same amount (even more than 20%) I would not worry about it. You are looking for a difference from the norm for no reason. An exception is strong cross wind will make the tires on the downwind side go up more than normal or the sun shinning on one side.
I knew I had something wrong when both of my tag tires increased 20% (10% is the norm for them) on a rainy/cool day while the drive tires did not. The pressure regulator was putting in over 100 psi (should be 45) to the tag airbags overloading the tag tires. Fortunately just turning the adjustment back and forth got it to start regulating the pressure again.

My TST tire monitors came with instructions to set them at 10% under and 20% over. So far that has worked fine. They also said to set the temperature at 158 degrees. Michelin states that tires can stand 180 degrees before they start being damaged.