I geeked out on this last summer while figuring out a new insert’s sweet spot for pressure and noticing that my pressure reading was always higher the top of 1000m laps and lower at the bottom.
Absolute pressure inside your tyres isn’t changing with elevation - it’s a sealed system with the volume of the tyre and number of air molecules inside it remaining unchanged. It’s to do with how pressure gauges work; they account for ambient air pressure which does change on a slight curve with altitude, which I simplified to 0.45psi per 1000ft for the elevations I ride. So pretty much 1.5psi per 1000m.
If you’re setting your pressure at home and are ignoring temperature changes don’t worry about it. But if you’re measuring pressure on the trail 1500-2000m higher than home, for example because it warmed up a lot since you set pressures in the morning or because you flatted, accounting for this is important.
[This is worth a read](https://www.coyoteents.com/compensating-tire-pressure-readings-for-elevation/ "COMPENSATING TIRE PRESSURE READINGS FOR ELEVATION"), and there's a calculator for what you should set at elevation (feet only, no metres) which confirms my rounded calculations well enough.
EDIT: Looks like that link above is dead. Shame, the calculator worked great. I was looking it up for reference while riding at lower elevations on a trip. [Here](https://www.coyoteents.com/tire-pressure-gauges-work/) is that website's page on how pressure gauges work if anyone is interested, and [here](https://www.sensorsone.com/altitude-pressure-units-conversion/) is a chart of atmospheric pressure to elevation - look up atmospheric pressure at your reference elevation (eg. 14.7psi at sea level) then look up atmospheric pressure at higher elevation (eg. 12.7psi at 4000ft) then compensate that difference if you're resetting pressure at altitude. So in this example you would set 2psi higher at 4000ft to have the same pressure that you would set at sea level. As I posted originally I simplified to 0.45psi change per 1000ft (1.5psi per 1000m) for the elevations where I live and ride regularly (1000-2500m) but it looks like this will get you within 0.1psi anywhere. Enjoy, nerds!
Feb. 11, 2022, 12:18 a.m. - Lornholio
I geeked out on this last summer while figuring out a new insert’s sweet spot for pressure and noticing that my pressure reading was always higher the top of 1000m laps and lower at the bottom. Absolute pressure inside your tyres isn’t changing with elevation - it’s a sealed system with the volume of the tyre and number of air molecules inside it remaining unchanged. It’s to do with how pressure gauges work; they account for ambient air pressure which does change on a slight curve with altitude, which I simplified to 0.45psi per 1000ft for the elevations I ride. So pretty much 1.5psi per 1000m. If you’re setting your pressure at home and are ignoring temperature changes don’t worry about it. But if you’re measuring pressure on the trail 1500-2000m higher than home, for example because it warmed up a lot since you set pressures in the morning or because you flatted, accounting for this is important. [This is worth a read](https://www.coyoteents.com/compensating-tire-pressure-readings-for-elevation/ "COMPENSATING TIRE PRESSURE READINGS FOR ELEVATION"), and there's a calculator for what you should set at elevation (feet only, no metres) which confirms my rounded calculations well enough. EDIT: Looks like that link above is dead. Shame, the calculator worked great. I was looking it up for reference while riding at lower elevations on a trip. [Here](https://www.coyoteents.com/tire-pressure-gauges-work/) is that website's page on how pressure gauges work if anyone is interested, and [here](https://www.sensorsone.com/altitude-pressure-units-conversion/) is a chart of atmospheric pressure to elevation - look up atmospheric pressure at your reference elevation (eg. 14.7psi at sea level) then look up atmospheric pressure at higher elevation (eg. 12.7psi at 4000ft) then compensate that difference if you're resetting pressure at altitude. So in this example you would set 2psi higher at 4000ft to have the same pressure that you would set at sea level. As I posted originally I simplified to 0.45psi change per 1000ft (1.5psi per 1000m) for the elevations where I live and ride regularly (1000-2500m) but it looks like this will get you within 0.1psi anywhere. Enjoy, nerds!