Adjusting tire pressure because it's cold outside

 I ran across a series of posts on an RV forum on the need to adjust tire pressure because of changes in ambient temperature. 

First lets be sure we all are using the same definition for "Ambient temperature" For our purposes we should consider it "the outdoor air temperature in the shade".

An RV owner noticed that his tire pressure had dropped based on his TPMS readings, almost 7 psi when the temperature dropped to the 40's. He did not provide his "normal" pressure or temperature. But he did claim " But within 10 driving miles they are back to normal pressure." He asked if he needed to add air, when it is cold, to bring pressure back to normal. There followed over 100 posts which is not uncommon when tire pressure is the topic. As expected, some offered correct information on the need to adjust pressure to accommodate the normal drop when it gets cold or increase in pressure when the ambient temperature rises. This immediately lead to various suggestions on the amount of adjustment needed.

I was heartened to read the comments from many, that knew it was important to adjust tire pressure to compensate for changes in ambient temperature. There were a couple of people that thought there was some special temperature for adjusting pressure. Replies to these people were quick and correctly pointed out that there was no special temperature such as 68F or 72F when tire inflation could be set but that a tire pressure can be set when the tire was not "warmed" by being in sunlight or having been driven on more than a mile.

Adjusting pressure when traveling

A few offered information on how to calculate the amount of air pressure that needed to be added but this concept quickly into discussions of Pressure vs Temperature formulas.

Bottom Line:

Tire inflation pressure will change about 2% for each change in Ambient Temperature of 10°F (6°C).

I advise that you use your TPMS to check inflation at the start of each travel day. This might be in the Morning but whenever you check you need to ensure that no tire has been warmed from being in direct sunlight for the previous couple of hours. Then set your tire inflation when the tires are at Ambient Temperature. It's just that easy.

There are charts on the Internet that cover how to adjust tire inflation under extreme conditions such as the RV being a garage heated to 65°F but the outdoor Ambient is way below freezing. I posted a chart last March but I bet that there are not many RV owners that need this information.

##RVT1028

Another question on "Cold Inflation" vs your "Set Pressure"

Got this question from a reader of an RV Forum:

Thank you for all of your informed comments regarding proper tire care. I need one clarification. I have always considered the cold psi on the side of my 22.5 RV tires to be the minimum to carry the maximum rated load, but have assumed that psi was also the maximum COLD psi the tire should see. From your recent post, am I to understand that unless the tire states that it is the maximum cold pressure, I can exceed it by 5-10 psi?
Thank you for your time, Doug

My Answer:

The wording on tire sidewall IMO was written by lawyers, not engineers or users. Info on the sidewall is the inflation needed to support the Max load.  The difficulty is that few understand that the pressure changes with temperature and the only meaningful pressure, measurement is when the tie is "cold". This still confuses some because some want to apply Chemistry Lab practice of adjusting to theoretical 72.5°F when what "cold" really means for tires is at "Ambient Temperature" and does not include any pressure build-up"   In real life terms this means "Not warmed by being driven on or in direct sunlight for the previous 2 hours"

Now we need to address what is meant by "Cold Inflation" vs the psi to set your tires to or what I like to call your "Set pressure".

I like to suggest the "Set Pressure" for motorhomes to be the minimum needed to support the maximum load on the tires PLUS 10% inflation. 

RV trailers are different because of their Interply Shear problem.

For RV Trailers,  I would like to see a minimum of +15% load capacity over the measured heaviest loaded tire, with +20% to +25% Reserve Load capacity being better. Sadly most RV trailers come with tires that provide +0% to +10% load capacity vs GAWR
NOTE: I am not even addressing the tendency for most RV owners to overload their tires.
So for trailers I try and simplify:
- To lower, but not eliminate the Interply Shear problem I suggest the "Set Pressure" when the tires are "Cold" to be the pressure on the tire sidewall. BUT I still want trailer owners to confirm they have at least 15% "Reserve Load" over their measured scale reading.

##RVT962