RV Tire Safety
with RV tire expert Roger Marble
Here was a recent post:
“I am looking for a formula for adjusting ‘cold inflation’ pressures for large tires. I have read that for smaller (car) tires the recommendation is 1 psi per 10 degrees F. What is also not clear is what the assumed ‘cold’ temperature is. Is it assumed to be 65 F, for example? So if we are sitting at 25 F, what is the adjustment?”
Here’s my reply:
Here is my blog post that explains “Cold Inflation.”
We are not in high school chem class so there is no adjustment needed to get to “standard” temperature.
If you are running a tire pressure monitoring system (TPMS), which you should be, you will soon recognize the normal range of temperatures and pressure variation in your tires.
I would not worry about adjusting inflation pressure if today’s expected temperature is +/- 10 F from yesterday. After all, you should be running +10% over the inflation you need to support the measured load.
Let’s say your minimum needed is 90 psi. Adding 10% to that means your cold inflation (“cold” meaning the tire was not driven on or in direct sunlight for the previous two hours) should be 99 psi, so you round it to 100 psi. All is good.
The next day’s weather is 20 F colder. That would mean the tire inflation will drop 4% (2% per 10 F change).
So now your tires would be 96 psi, but since you have to stay above 90, I would say there is no reason to worry about adding air – just enjoy life and head out.
If the next day your weather went up 10 F instead of down, that means your “cold” inflation would go up 2% from the 96 psi, or to about 98 psi. Again, you don’t need to do anything.
I would not worry about lowering the pressure until I saw pressures 10 psi above the pressure molded on the tire sidewall. When you get to that point you could bleed off a few psi – but remember if the weather gets cold over the next couple of days you will be needing to add air again.
Don’t get your shorts in a bunch about inflation. Just watch your TPMS and you will learn what is normal for your coach.
Read more from Roger Marble on his blog at RVtiresafety.net.
Check out this great TPMS: The New TireTraker™ TT-500 with a Lifetime Warranty.
I think we are ‘overthinking’ this whole tire pressure thing. I just check my tire pressure every morning (that we’re actually on the road) before the sun heats stuff up.
I hadn’t heard the 110% of minimum rule before – I’ve gone with sidewall cold psi. I can say you have to watch pressure clearly if you’re a long hauler on a North-South route. My new TPMS seemed possessed until I manually checked, and 700 miles south really had risen +70F ambient and about 10psi… reverse that on the return trip.