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Friday, November 26, 2021

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.


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