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post #1 of 17 (permalink) Old 03-05-2019, 10:13 AM Thread Starter
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What's the correct air pressure?

I'm on my 3rd set of Michelin MXM4 tires. I think this is the stock tire. I've been adding about 40 pounds but the sticker in the door jam indicates 32 pounds.


What's the correct pressure?

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post #2 of 17 (permalink) Old 03-05-2019, 10:59 AM
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Whatever the door jam states.

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post #3 of 17 (permalink) Old 03-05-2019, 01:00 PM
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Not necessarily. The door jamb is for the OE tires and a "typically" loaded vehicle.

Higher pressure means:

1. Center of the tire may wear more than the outsides. (Bad if that happens, since it impacts tire life.)
2. Lower rolling resistance (sidewall flex) in normal driving = slightly better fuel economy.
3. Possible harsher ride.
4. Lower tire heating during highway driving (less flex = less heat generated)

Lower pressure means:

1. Outsides may wear more than the center. (Bad if that happens, since it impacts tire life.)
2. Higher rolling resistance (sidewall flex) in normal driving = slightly worse fuel economy.
3. Possible softer ride.
4. Higher tire heating during highway driving (more flex = more heat generated)

The "sweet spot" printed on the door jamb assumes many things; the actual load range of the tires (yours may differ from factory delivery), the load in the vehicle and how it's distributed, your normal highway speed and thus level of heat generated, etc. Those assumptions may or may not be true for you.

The sidewall on the tire has printed the maximum load per-tire at the stated maximum rated inflation pressure. That's an absolute limit that should never be exceeded. The closer to that load limit you get with less inflation the higher the risk of a heat-related failure, which can be catastrophic (that's to be avoided!) I am not aware of any tire manufacturer that publishes an easily-accessible load-derating curve for their tires, and IMHO that sucks -- and you can bet there is one since HEAT is the primary issue as load rises.

In general you probably want to consider the door sticker the LOWER limit and the sidewall pressure the UPPER limit -- and an absolute. Where you run between those two points depends on many things; if you get abnormal wear patterns (center or edges wearing more than the other) you're definitely running too low or high tire pressure, but if not then the trade-offs are more complex.

I typically (for everyday driving) run my rear tires ~2psi below my fronts on most FWD vehicles as I usually have little or nothing in the trunk and nobody in the rear seats. If I'm going to be traveling somewhere with a full set of suitcases and such in the back, then I run equal pressure. I favor the higher end of the pressure scale as well but I've not run into the center of the tires wearing before the sides; if I did I would cut it back since giving up tire life for a tiny increase in MPG, never mind the likely impact on traction, is not worth it.
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post #4 of 17 (permalink) Old 03-05-2019, 01:05 PM
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Quote:
Originally Posted by tickerguy View Post
Not necessarily. The door jamb is for the OE tires and a "typically" loaded vehicle.

Higher pressure means:

1. Center of the tire may wear more than the outsides. (Bad if that happens, since it impacts tire life.)
2. Lower rolling resistance (sidewall flex) in normal driving = slightly better fuel economy.
3. Possible harsher ride.
4. Lower tire heating during highway driving (less flex = less heat generated)

Lower pressure means:

1. Outsides may wear more than the center. (Bad if that happens, since it impacts tire life.)
2. Higher rolling resistance (sidewall flex) in normal driving = slightly worse fuel economy.
3. Possible softer ride.
4. Higher tire heating during highway driving (more flex = more heat generated)

The "sweet spot" printed on the door jamb assumes many things; the actual load range of the tires (yours may differ from factory delivery), the load in the vehicle and how it's distributed, your normal highway speed and thus level of heat generated, etc. Those assumptions may or may not be true for you.

The sidewall on the tire has printed the maximum load per-tire at the stated maximum rated inflation pressure. That's an absolute limit that should never be exceeded. The closer to that load limit you get with less inflation the higher the risk of a heat-related failure, which can be catastrophic (that's to be avoided!) I am not aware of any tire manufacturer that publishes an easily-accessible load-derating curve for their tires, and IMHO that sucks -- and you can bet there is one since HEAT is the primary issue as load rises.

In general you probably want to consider the door sticker the LOWER limit and the sidewall pressure the UPPER limit -- and an absolute. Where you run between those two points depends on many things; if you get abnormal wear patterns (center or edges wearing more than the other) you're definitely running too low or high tire pressure, but if not then the trade-offs are more complex.

I typically (for everyday driving) run my rear tires ~2psi below my fronts on most FWD vehicles as I usually have little or nothing in the trunk and nobody in the rear seats. If I'm going to be traveling somewhere with a full set of suitcases and such in the back, then I run equal pressure. I favor the higher end of the pressure scale as well but I've not run into the center of the tires wearing before the sides; if I did I would cut it back since giving up tire life for a tiny increase in MPG, never mind the likely impact on traction, is not worth it.
Ok, I stand corrected. yeah what Ticker says is absolutely correct.

Seriously though for mere mortals who won't "baby" their cars like many of us, wouldn't the door jam be the "most" reasonable, safest pressure to follow?

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post #5 of 17 (permalink) Old 03-05-2019, 01:13 PM
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FWIW, i would also think lower pressure would make wheels more exposed to pothole damage, which is one of the reasons I am running my winters about 1-2 psi higher. Of course this can also affect snow traction since the contact patch may not be optimal for the best snow traction.

But I'll wait for ticker to correct me.

Seriously though should have waited for ticker to comment on this thread first since his explanation is always the most thorough.

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post #6 of 17 (permalink) Old 03-05-2019, 01:27 PM
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The in-snow traction compromise from higher pressure may well be true as well -- but the impact in that regard should be quite small. Not zero though!

What you really want to know in terms of where the *minimum* safe pressure is you don't have -- the load derating curve for the tire with pressure (down from the maximum on the sidewall) AND the weight each wheel carries. Without that it's a guess. The manufacturer doesn't want to be sued so I would treat the door jamb numbers as *minimums*; they likely have the derating curve for the OE tires (but you don't) and thus you're likely safe running there -- but perhaps not below that point. Anywhere between door jamb and tire sidewall pressures are going to be safe, but "safe" and "best option" are two different things.

If the ride quality is unacceptable at a higher (but at or under sidewall rating) pressure then obviously you don't want to run there either. However, assuming tire wear is even and ride quality acceptable higher pressure will result in slightly better fuel economy and less tire heating -- and both are good things.

The specific tires matter quite a bit when it comes to this as well, especially as regards ride quality change with inflation pressure.
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post #7 of 17 (permalink) Old 03-05-2019, 01:50 PM Thread Starter
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Every little imperfection in the road feels like a hammer blow to the car. I'm dropping the pressure.

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post #8 of 17 (permalink) Old 03-05-2019, 02:03 PM Thread Starter
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It's so bad that I'm thinking about new wheels that would allow for a higher profile tire.
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post #9 of 17 (permalink) Old 03-05-2019, 02:14 PM
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I want to do the same with my Speed. I hate the 18 inch wheel/low profile tire combo, I want some more rubber.
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post #10 of 17 (permalink) Old 03-05-2019, 03:07 PM
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Quote:
Originally Posted by TampaBay6 View Post
It's so bad that I'm thinking about new wheels that would allow for a higher profile tire.
One of the reasons I bought a "Sport" (as opposed to Touring) is that I did NOT want the low-profile tires and wheels.

I had low-profile tires and wheels on a Volvo 850T a number of years ago. Never again will I own them on a daily-driver. Yes, they're more-precise in their handling and road feel. The harshness is IMHO simply not worth it, never mind the radically-increased risk of not only tire but *wheel* damage from potholes and similar. Oh, and the added expense too; the tires, despite having LESS material in them always appear to cost more -- usually by a lot too.
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