[msej449]

Does altitude make any difference to the tyre pressure? Our car lives literally at sea level. But currently it's sitting at 1500m (5,000ft) in the Swiss Alps, and will be driven as high as 2500m (8,200ft) in the next couple of weeks.

Having done a web search, if it's 32 PSI at sea level at 20C then it should be 26.79 PSI at 1500m - around 5 PSI less. At the St.Bernard Pass (2,469m) the equivalent would be 24 PSI, which is a big difference.

Do forum members make any tire pressure adjustments when they travel into the mountains?

It's probably infeasible to keep resetting the pressure every time we go down to the valley to shop, but for our trip into Italy next week, over the St. Bernard Pass, and to the Aosta Valley, it may be worth dropping the tire pressures?

[overcoil]

I guess as long as the tire pressure (increase at altitude if filled at lower altitude) doesn't exceed the maximum tire pressure on the sidewall ( maximum sidewall pressure probably near 50 psi ) your fine.
If you were traversing rocks at 50 psi then that's may not be good, but you'd never see 50psi if you use manufactures pressures at whatever altitude you fill and then travel to.

[dradernh]

Originally Posted by msej449

Do forum members make any tire pressure adjustments when they travel into the mountains?

I haven't. For perspective, my driving on highways with significant elevation changes has always been steady-state cruising at the speed limit.

From experience, my principal example in the States is Interstate Highway 70 (I-70) between Denver, Colorado and Grand Junction, Colorado. The the low elevation on this route is 4,585' / 1,398m, and the high elevation is 11,942' / 3,640m.

This route is traveled by a large number of passenger cars and trucks every day, and I would be surprised if any of them are adjusting tire pressures as they travel the route.

This succinct and informative piece by the experts at Tire Rack provides specific and useful information on the subject that you may find useful: https://www.tirerack.com/tires/tiret...jsp?techid=167

[aerobod]

Due to atmospheric temperature variation of 6.5C per 1000m, much of the pressure change due to altitude is offset. With a nominal 1.01 bar higher pressure inside the tyre at sea level compared with the tyre pressure gauge reading and a gauge reading of 2.40bar (35PSI) at 25C (298K), constant pressure times temperature for the fixed tyre volume is maintained for air not saturated with moisture, leading to variation of measured pressure and temperature with altitude as follows:

0m, 1.01bar atm press, 2.40bar measured press, 298K
1000m, 0.90bar, 2.44bar, 291.5K
2000m, 0.79bar, 2.47bar, 285K
3000m, 0.69bar, 2.50bar, 278.5K
4000m, 0.60bar, 2.52bar, 272K

So sea level to 4000m gives a variation of 0.12bar, or 1.8PSI, not really enough to worry about. We live at 1250m, have nearby paved roads up to 2200m and a vacation place at 350m, so see less than 0.1bar measured tyre pressure over the places we visit regularly. There is a lot more tyre pressure variation due to our -35C to 35C seasonal range of temperatures.

One of the more noticeable affects on the vehicle if it has absolute tyre pressure monitoring sensors is the inaccuracy with altitude due to not having an external pressure reference. Our Macan S has this type of sensor, so at our normal altitude of 1250m where we live we see readings 0.1bar to 0.2bar below the tyre gauge measurement

[msej449]

Thanks for all the advice. After further reading, I think I may have misunderstood the physics. Inasmuch as it's the outside pressure that drops, so this will increase the measured pressure of the tire. e.g. if it's, say, baseline 32 PSI at sea level then it'll be 32+'x' PSI higher at 1500m, so I'll have to drop it by 'x' to get back to the baseline - you don't reduce the baseline pressure itself. And from what everyone says, the 'x' is going to be so relatively trivial that it's not worth the adjustment anyway, especially in comparison to parallel changes in load (it was loaded-up to get here, but is now unloaded), and temperature (which has ranged from 15°C [59°F] to 33°C [92°F]).