17" Tire pressure question?
Last edited by MCF; Jun 7, 2013 at 12:49 PM.
I would start with the factory settings and adjust from there. I've always found factory settings to be a bit on the low side as the manufacturer needs to walk a fine line between handling, tire wear and comfort.



I am a 2012 luxury diesel...
I set my tires to the factory settings and drive for a week. Then I bump them up by about 3 psi and drive for another week. If my teeth haven't been jarred from my mouth then I leave it there.
By law, the TPMS sensors must alert at 25% under the required pressure, so if the recommended pressure is 40psi, then the sensors will alert around 30psi. 15-20% under the recommended pressure will lead to increased tire wear and more than 25% can become dangerous. Around 30% or so low and the pressure becomes critical, possibly causing internal (invisible) tire damage.
Knock on wood, but I've also never had a catastrophic tire failure - although I've had my slow leaks and nails - in part because I check pressure and tire condition frequently and change my tires at 4/32nds.
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I set my tires to the factory settings and drive for a week. Then I bump them up by about 3 psi and drive for another week. If my teeth haven't been jarred from my mouth then I leave it there.
By law, the TPMS sensors must alert at 25% under the required pressure, so if the recommended pressure is 40psi, then the sensors will alert around 30psi. 15-20% under the recommended pressure will lead to increased tire wear and more than 25% can become dangerous. Around 30% or so low and the pressure becomes critical, possibly causing internal (invisible) tire damage.
Knock on wood, but I've also never had a catastrophic tire failure - although I've had my slow leaks and nails - in part because I check pressure and tire condition frequently and change my tires at 4/32nds.
Regarding TPM. I disagree with above comments. The TPM is not set for any exact tire pressure. (i.e.40 lbs). Put in whatever tire pressure you want. You then need to ACTIVATE the TPM at that pressure. Low pressure warning system will take over from there. Any time you change tire pressures the TPM must be reset to that new pressure. This is done from the steering wheel buttons. If all else fails read the book! If you do not reset, then the TPM will react to the last reset pressure.
Regarding TPM. I disagree with above comments. The TPM is not set for any exact tire pressure. (i.e.40 lbs). Put in whatever tire pressure you want. You then need to ACTIVATE the TPM at that pressure. Low pressure warning system will take over from there. Any time you change tire pressures the TPM must be reset to that new pressure. This is done from the steering wheel buttons. If all else fails read the book! If you do not reset, then the TPM will react to the last reset pressure.
In my old S5 it recommended 39 psi front and 36 psi rear. All 4 tires were the same size (255/35/19). The weight distribution of the car required more pressure in the front to prevent the tires from deforming differently than the tires in the rear. 39 all around was too harsh and tail happy, 36 all around was too sloppy in the corners. I tried higher and lower, different ratios, but the car always felt best at 39f/36r COLD pressure. I found in 5 years of ownership that even when I went from the summer tires to 225/45/18 tires for winter, that the car felt best at these pressures.
For reference:
Pressure of a Gas = (amount of gas * Gas Constant * Temperature) / Volume.
In my old S5 it recommended 39 psi front and 36 psi rear. All 4 tires were the same size (255/35/19). The weight distribution of the car required more pressure in the front to prevent the tires from deforming differently than the tires in the rear. 39 all around was too harsh and tail happy, 36 all around was too sloppy in the corners. I tried higher and lower, different ratios, but the car always felt best at 39f/36r COLD pressure. I found in 5 years of ownership that even when I went from the summer tires to 225/45/18 tires for winter, that the car felt best at these pressures.
For reference:
Pressure of a Gas = (amount of gas * Gas Constant * Temperature) / Volume.[/QUOT
Have 18" tires. This is my 29th Mercedes over 40 yrs so I am reasonably familiar with the features of MB.I know the diff from a Chevy. Yes the filler door specs the same pressure all around. I stated what I do and have found what works for me. You are free to do what you want. I am not out to convert anyone. I am agreeable to listening to other views;but pls no lectures.




Regarding TPM. I disagree with above comments. The TPM is not set for any exact tire pressure. (i.e.40 lbs). Put in whatever tire pressure you want. You then need to ACTIVATE the TPM at that pressure. Low pressure warning system will take over from there. Any time you change tire pressures the TPM must be reset to that new pressure. This is done from the steering wheel buttons. If all else fails read the book! If you do not reset, then the TPM will react to the last reset pressure.
I had my rear pressure set at around 36 psi with the Conti crap tires and they totally wore out in the middle. I put new tires on and keep pressure at around 33 psi and I'm trying to monitor the wear. Don't know yet how the wear is across the width of the tire.
I always drive with no load in the car and the 36 psi for the rear seems way too high.
I always drive with no load in the car and the 36 psi for the rear seems way too high.




But for curiosity, what would you think is wrong other than tire pressure when the rear tires both wear severely in the middle with no sign of other than normal wear at either inside or outside edge of the thread?
What would that "something else is wrong" be?
Last edited by Arrie; Jun 12, 2013 at 04:29 PM.




One way to check is to periodically measure tread wear to make sure it is even.
My 2010 manual says use higher pressure for fully loaded or driving over 100 MPH!
Forget which way, but too much wear on inside and outside indicates either too low or too high a pressure.
Once you decide on a pressure, don't forget to reset TPMS.

Increased outer tread wear (both sides) indicates UNDER-inflation.
Right CEB?
But for curiosity, what would you think is wrong other than tire pressure when the rear tires both wear severely in the middle with no sign of other than normal wear at either inside or outside edge of the thread?
What would that "something else is wrong" be?
Remember that the psi recommendations on your door jamb are a compromise between handling (more psi is better) tire wear (more is better) and comfort (lower is better). On most luxury cars, the ride is weighted towards the comfort side so the sticker is actually below what it really should be.
In any case, 3psi will not show appreciable center wear but you must remember that the TPMS will not warn of overinflation - only underinflation and many cars come from the factory with extremely overinflated tires.
I'll ask one of the guys down the hall to do some research - or they may know off the top of their head as to what else could cause this - but if it was overinflation then it was more than 3psi.




Remember that the psi recommendations on your door jamb are a compromise between handling (more psi is better) tire wear (more is better) and comfort (lower is better). On most luxury cars, the ride is weighted towards the comfort side so the sticker is actually below what it really should be.
In any case, 3psi will not show appreciable center wear but you must remember that the TPMS will not warn of overinflation - only underinflation and many cars come from the factory with extremely overinflated tires.
I'll ask one of the guys down the hall to do some research - or they may know off the top of their head as to what else could cause this - but if it was overinflation then it was more than 3psi.
I tried to keep my rear tires at 36 - 37 psi but obviously this way too much. I have new tires now and try to keep them 33 - 34 psi.




