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The Tire Rack's Tire & Wheel Forum
Use this forum to discuss anything in relation to wheels to tires to offsets.
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Old 11-14-2016, 09:12 PM   #1
bmwstephen
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Spare Tire PSI?

Does anyone know the proper psi output for the spare tire? I figured it be different (higher if anything) because it is not mounted on the car with load.
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Old 11-14-2016, 09:23 PM   #2
rustypigeon
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I usually fill my spare to near the max psi listed on the sidewall. If I get a flat, I will then deflate it to the psi listed on the door jamb. I always keep a pencil gauge in the trunk tool kit.
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Old 11-14-2016, 09:27 PM   #3
saxual328ci
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Since its a full size tire, I keep it around the same as the other tires. Or when I put it on the car, I will add more air if needed.
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Old 11-14-2016, 09:44 PM   #4
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I fill my spares to the max tire inflation listed on the tire, usually around 45 PSI as I recall?

This is because you may go YEARS without checking the tire pressure or using the tire so you want to allow for some air leakage and still be able to install the tire and drive on it!
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Old 11-14-2016, 09:58 PM   #5
Kpriest
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Quote:
Originally Posted by bmwstephen View Post
Does anyone know the proper psi output for the spare tire? I figured it be different (higher if anything) because it is not mounted on the car with load.

Common misconception, the air pressure depends on volume, temperature, and the amount of air in the tire. This can be proven by the ideal gas law. Load does not factor in to the air pressure.

The max PSI rating on my tires is 50. I keep my front at 35psi and rear at 42psi according to the drivers side ID card with the recommended pressures. I keep my spare at 40psi and check it whenever I service the car (oil change, etc..).
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Old 11-14-2016, 10:25 PM   #6
steveintheburgh
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the Spares max is 60PSI - filled it to that last week when I discovered it's valve stem had given out and had it replaced
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Old 11-14-2016, 10:29 PM   #7
Kpriest
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Originally Posted by steveintheburgh View Post
the Spares max is 60PSI - filled it to that last week when I discovered it's valve stem had given out and had it replaced

Is that a full size spare?
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Old 11-15-2016, 12:21 AM   #8
bk_856er
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My '03 325XiT has a donut/skinny/steel/cheapo/hi-pressure/max. 45mph spare to save cost and weight.

A family member's loaded '03 325XiT has a full size spare with an alloy rim that matches the other 4.

Go by the sidewall and check periodically. Not all BMW's have the same type of spare tire!

BK
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Old 11-15-2016, 06:46 AM   #9
paraklas
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Originally Posted by Kpriest View Post
Is that a full size spare?
60 PSI (4 bar) is the pressure rating for the space saver, not the full size.
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Old 11-15-2016, 07:24 AM   #10
BaliDawg
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Quote:
Originally Posted by Kpriest View Post
Common misconception, the air pressure depends on volume, temperature, and the amount of air in the tire. This can be proven by the ideal gas law. Load does not factor in to the air pressure.

The max PSI rating on my tires is 50. I keep my front at 35psi and rear at 42psi according to the drivers side ID card with the recommended pressures. I keep my spare at 40psi and check it whenever I service the car (oil change, etc..).
But doesn't the car's weight compress the tire's volume, thus increasing air pressure within? Sit on a balloon and it pops (at least with my fat ass).

PV = nrT
P = nrT/V => lower volume increases pressure
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Last edited by BaliDawg; 11-15-2016 at 07:25 AM.
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Old 11-15-2016, 11:08 PM   #11
Kpriest
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Quote:
Originally Posted by BaliDawg View Post
But doesn't the car's weight compress the tire's volume, thus increasing air pressure within? Sit on a balloon and it pops (at least with my fat ass).



PV = nrT

P = nrT/V => lower volume increases pressure

Tires are not ridged, true. They are elastic and therefore the volume does change with load but that change is insignificantly small. A balloon is much more elastic than a tire due to the balloons thickness and lack of steel bands. If you do not believe me take your tire off check the air pressure then reinstall and check again.
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