red308

On our 2013 ML350 I got a warning message to "correct the tire pressure".

So I looked on the driver side B pillar and on the gas cap. The info labels are wildly different:

On B pillar: 45 psi cold

On gas cap: 32 psi cold

That is a radical difference. Did I miss something here? Does anyone have any authoritative info on what is going on here? Which is the correct answer?

bud.jones

I saw a review on YouTube (Car and Driver, I think) that said that the value on the gas cap was the correct one. I'll check and post the link. That was the only review that even mentioned the discrepancy. It's at the 3:15 mark in the video.

EDIT: It was actually Consumer Reports.

jadatis

You probably have low aspect ratio tires ( the 40 in fi 235/40ZR18 90W).
Those tires , so I discovered are to high calculated by the tire-makers in their maximum load. Also they are more sensitive to a sideward angle of the wheel to the road. For these factors you need higher pressure to compensate that, so the tires ( sidewals) are not damaged by driving alone.

Before 2000 they yust calculated the advice pressures with a formula , that I got hold of end 2007 and went running with.
After 2000 the tire-makers organisations discovered their mis-calculation, and did strange things to the plates on the car, like you now discovered.

So to calculate a save pressure for your tires , I need GAWR's( gross axle weight ratings) maximum speed , camber angle ( alighnment mosly only back when seen from behind wheels like this/-\ on the axle instead of this|-|).
this is for fully loaded and maximum speed of car.

If you want it for normal use, wich they also gave before 2000 in Europe, for 3 persons an a little load, I also need the Empty weight of car, and build ( motor in front or back) and how you load it ( persons and load) normaly, so I can estimate the axle loads. Also the maximum speed you sertainly wont go over in normal use ( lower speed can do with lower pressure)

From tires , exact sises and maximum load and tire-kind ( to determine the reference pressure Standard load or XL/Reinforced/Extraload), and speedrating (the W in the example above stands for up to 270km/<>155m/h).

So if we have all that , you can forget the old advices and have and advice wich is save to laws of nature.
But you wont be happy with it if you realy have those low aspect ratio tires,
Can be then that you need such a high pressure for savety, that comfort and gripp are totally lost. For that you need tires with a higher Aspect ratio.

red308

Thanks for posting the video. Yes, those are the offending labels. Just wrong. I am going to check with MBUSA about those.

The tires are the standard tires: 255/50R19. It should not be a rocket science exercise to figure out what pressure to use for these.

GregW / Oregon

Gas cap label is the correct one, unless you like riding on rocks. Very confusing for a lot of people.