Altitude and tire pressure - Mercedes Benz SLK Forum

Tires, Wheels, Brakes & Suspension Discussion and information regarding tires, wheels, brakes and suspension.

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#1 Old 08-15-2014
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Altitude and tire pressure

Tire pressure used to be simple. Every so often, find the gauge, check it when the car was cold, adjust. The SLK was supposed to make it simpler, once I figured out what buttons to press. I blame the dealer for not setting the initial pressure correctly: too high in front. (The placard inside the fuel door says 30psi in front, 36 in back.) I tried to fix it using my trusty gauge and quickly found out how complicated tire pressure was.

Probably everyone else knows, don't let a few pounds of air out without telling the car. Reset the system so the computer knows that there wasn't a sudden drop.

But then the car says 27 psi while my trusty gauge says 30 psi. Who do I believe? It turns out altitude is the culprit. The tire pressure sensors are reading internal tire pressure, while the gauge is external and affected by the altitude. It therefore reads about 2.6 psi higher than the car sensors.
I found a lot of conflicting information on what number to use. Some of it was geared towards people on vacation - don't go to the top of a mountain and let all the air out of your tires, you'll need it later. I asked a service advisor at the dealer, who said go with the car's numbers but could not say why. I found this after five pages on tire pressure in the manual:

The tire pressure values indicated by the onboard
computer may differ from those
measured at a gas station with a pressure
gauge. The tire pressures shown by the onboard
computer refer to those measured at
sea level. At high altitudes, the tire pressure
values indicated by a pressure gauge are
higher than those shown by the on-board
computer. In this case, do not reduce the tire
pressures.


I interpret that to mean that the important thing is to maintain the pressures at sea-level numbers. What does everyone else think?
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#2 Old 08-15-2014
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This is also my understanding: maintain tire air pressure at sea level.
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#3 Old 08-16-2014
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How do you "tell the car"? And just curious what altitude changes you might've been seeing?
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#4 Old 06-25-2015
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Use google maps and factor in an increase in tire pressure of around 1.74psi for every 1000m you climb?
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#5 Old 06-25-2015
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And in Colorado where Just1MoreDave lives, you can drive on pavement now all the way up Pike's Peak to about 14,000 feet, or about 4300 meters. Per Tricolour's figures, you should read about 7psi high at the summit. The Eisenhower Tunnel on I-70 is at about 11,000 feet, and so is the Athabasca Glacier on the Icefields Parkway in Alberta. Out west in North America, it's pretty easy to drive at some relatively high altitudes.
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#6 Old 06-25-2015
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On a hot summer day you can actually be a good bit higher, in terms of density alitude, than your geodetic height. I live at some 4000' but on a 90F summer's day I will be close to 7000', not only that but my HP will be closer to 220 than 302, that sucks. I just use a good quality digital tire gauge, and set the cold pressure to what the manual tells me. I have two good digital, and one good analog gauge, I do a cross check every now and then as a sanity check. I check my cars weekly, for correct tire inflation. About every two weeks at least one car will need a wee-bit of air. The wife's Boxster is the worse, not sure why?
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#7 Old 06-25-2015
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Quote:
Originally Posted by Beekster View Post
And in Colorado where Just1MoreDave lives, you can drive on pavement now all the way up Pike's Peak to about 14,000 feet, or about 4300 meters. Per Tricolour's figures, you should read about 7psi high at the summit. The Eisenhower Tunnel on I-70 is at about 11,000 feet, and so is the Athabasca Glacier on the Icefields Parkway in Alberta. =.
Arrr - "The Eisenhower Tunnel on I-70" will be the tunnel us Brit's pass through when heading to Vail & Breckenridge...
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#8 Old 06-25-2015
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Check Tires at Sea Level might be hard to do for People in the Alps and Colorado.
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#9 Old 06-26-2015
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Quote:
Originally Posted by Trev5 View Post
Arrr - "The Eisenhower Tunnel on I-70" will be the tunnel us Brit's pass through when heading to Vail & Breckenridge...
Yes, that is the one. It's a fun trip. You start going up, as expected. Then there is quite a lot of up. At some point you notice that the outside air temperature has dropped. You should see snow. Then the climb continues for a lot longer. You get used to using a lot of accelerator pedal. Soon after the tunnel, you start to go down, which doesn't look that steep until you see that you are rolling at 85mph. Heavy SUVs, excessively popular here, freefall down that hill burning up their brakes.


The eastbound part is Eisenhower. The official name for the westbound side is Johnson, but few people call it that.
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#10 Old 06-26-2015
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I take no responsibility for the correctness of the attached information.


Coyote Enterprises' Gauge altitude compensation calculator
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