Originally Posted by
Kapusta
When doing the above calculations, if you want to be more accurate, you need to use the actual pressure inside the tire, which is the pressure you see on your tire gauge, plus the ambient atmospheric pressures (around 14psi at sea level). The pressure you see on the gauge is actually measuring the difference between the pressure in your tire and the pressure outside.
As it turns out, when you are talking about high pressures (like 95 psi as we read it on our gauges) the effect on the results is negligible. But at very low pressures (like fat bike in the snow at 2-3 psi) it make a significant difference.
It’s not enough of a difference to matter if the pressure is gauge or absolute. It would make a difference if you are pumping the tire up and then changing altitude but that’s not what most people are going to do. The ambient pressure at a specific location will vary only a little. To use your fat bike example, a tire going from 3psi at -5°C (23°F) to 0°C (32°F) would see a pressure change of 3.1psi. Using the absolute pressure (17psi), the pressure change is 3.3psi. The change is small enough that it can’t be measured by bicycle gauges currently in use.
If the temperature change is from 0°C to 15°C (60°F), the pressure change is 3.2 psi gauge and 3.9 psi absolute. Not enough to make a difference.