DaveW68

Yes, the mechanics, especially non-Benz mechanics get very confused by the labels. The best recommendations for your truck are on the gas cap door. If you read the door carefully, you'll see that the higher PSI recommendations are for pulling a trailers (rear tires only) or driving over 100 mph (all tires).

N_Jay

Dave will hate me for this, but here is my recommendation. (This assumes the tires are the correct load rating for your vehicle as you typically have it loaded.)

Start with your typical use and the factory recommendations. (assume 32 for the discussion)
This is your minimum pressure.

Find out the tire manufacturers maximum recommended pressure (assume 51 for the discussion)
This is your maximum pressure.

Now apply these two general rules of thumb.
1) Never trust a tire gauge to be better than +/- 2 PSI (+/- 10 kPa)
So add this to your minimum and subtract it from your maximum.
You now have new minimum and maximum. (New minimum 34 and Maximum 49)

2) Unless you are a profe3ssioal race driver you will not be able to feel any difference less than 2 PSI
So, when you make changes, make 2 PSI changes and give yourself enough time to figure out if it is better or worse.

Now here is your option;
If you carry heavy load, drive fast for long distances, want the best handling and tire life, start at the top.
If you drive normal, and appreciate ride comfort start at the bottom.

Fill your tires COLD.
Drive a bit and see how it feels.
Give it at least 1 hour of steady driving before raising the pressure (starting low) , but you can drop the pressure at any time (Starting high)
Note how it feels
Lower or raise the pressure 2 PSI .
Drive a bit and see how it feels.
Give it at least 1 hour of steady driving before raising the pressure (starting low) , but you can drop the pressure at any time (Starting high)
When it feel right note that pressure.
Lower or raise the pressure another 2 PSI .
Drive a bit and see how it feels.
Give it at least 1 hour of steady driving before raising the pressure (starting low) , but you can drop the pressure at any time (Starting high)
Note how it feels.
Keep Lowering or raising the pressure 2 PSI .
When you stop getting any better, or starts getting worse note that pressure.
You now know the top and bottom of the range you like for your driving.
Set the pressure anywhere in that range. (I like the top, as tires lose air, never gain air.)

Now, if you want you can independently take the fronts up and/or down 2PSI at a time to see if there is a better pressure for the front, then do the same with the back.

N_Jay

Don't forget that an weight equalizing hitch also load the fronts, bit I am sure that is on the sticker too.

DaveW68

Well, there are 2 points of view.....the MB engineers who developed and built these trucks......and yours. At least folks have a choice on whose advice they'd like to follow.

Interestingly, I had 4 all-weather tires mounted on 19" wheels in late November for winter use. Pep Boys did the installation and put the fronts at 38 and the rears at 46. I had no idea that they set the pressure to this until the first snow came and I didn't think the traction was as good as it should have been. I checked the pressure and noticed the inflated numbers. I immediately put them all to 34 PSI and the handling in the snow and ice got significantly better. It also gave a much more comfortable ride. Same thing when it came to rotating my stock 21" wheels in the summer. I noticed a much harsher ride and worse handling in the rain after the tire rotation. The tires had been set at similar pressures by the guys doing the tire rotation 36 & 44 IIRC. I also put them at 34 PSI and handling in all weather conditions improved as well as the ride. Dry roads had a marked improvement as I wasn't constantly spinning all 4 wheels when giving it some gas from a stop (I've got an OE Tuning tune on my truck).

N_Jay

Oh no!

There are LOTS of opinions on tire pressure.

Now let's discuss the best oil.

And let me guess, you throw away all food past it's "Use By" date?

dragosdor

What about the pressure gauge accuracy? My Longacre liquid filled tire pressure gauge shows 35psi all around, while the on board TPMS shows 3psi higher on a consistent basis. Which one should I believe? P.S.: also the digital display Longacre shows the same 35psi. Also the front right sensor is always 1-2psi higher than the others.
2013 X166 GL450

DaveW68

I'm not familiar with that tire gauge. But my good old fashioned manual gauge is always spot on or within 1 psi with the TPMS sensors. The most out of whack readings seem to come from the tire shops who use the spot reader when filling the tires after a tire swap or repair. In my experience, those almost always read 3-5 lbs low. The same goes for the readers at the air fillers at gas stations.

N_Jay

An important lesson from an Engineering Professor: "A man with 2 watches never knows what time it is!"

DaveW68

Lesions are usually pretty bad......and in many cases, deadly.

N_Jay

I said and Engineering Professor not a English Teacher!
It could be just a scab? Why are you picking at it?

DaveW68

Have you ever seen pictures of meth heads who are always picking at their face to get to the lesions on their brains?

Scary stuff.

N_Jay

True, but there are many lesions that are insignificant. (and far from deadly).

DaveW68

Like engineering professor lesions? I see your point.

N_Jay

I'm kind'a sure you didn't.

SaniDel

Took in our 2015 GL550 for service and took out a 2019 GLS450 loaner with 35 miles on the odometer, which says something about demand for the GLS if they are being used for loaners rather than sold!

Anyway, took a picture of the fuel filler door, which now has a specific rating for the 295/40 R21 "Extra Load" ... notice that the minimum cold tire pressure is 34 PSI front and 35 PSI rear ... 2 and 3 PSI more than the standard cold tire pressure.

DaveW68

I wonder why the 295/40R21 specify "Extra Load" when the other sizes don't? I always thought that all tires that went on these trucks were supposed to be XL's based on the weight of the truck. Perhaps the 450's don't need XL tires when running 19 or 20" wheels? It's probably best to consult the owners manual for clarification.

BTW, those are not "minimum" PSI numbers. They are the recommended cold tire pressures to get the longest life out of the tires (even tread wear across the tire) with the safest and most compliant ride across all driving conditions.

N_Jay

Don't you mean; "those are not "minimum" PSI numbers. They are the recommended cold tire pressures to get the best compromise between, the longest life out of the tires (even tread wear across the tire), and most compliant ride, while still within the safe range, across typical driving conditions.

DaveW68

No

N_Jay

LOL.

Supporting evidence?

DaveW68

Call MB in Germany and ask to speak with the GL engineers. Be sure to get a German to English translator and tell them how you know better than they do about PSI. It should be a fun and interesting conversation.

Or you can hope that you have a knowledgeable service adviser at your local dealership and see what they have to say.

N_Jay

Given there are different pressures for lightly loaded and heavily loaded, and different pressures for below and above 100 MPH,
1) how can you believe that any one number is not a compromise of many factors.
2) how can you believe that the one number is near optimum?

DaveW68

Go ahead and run 50 PSI. Or perhaps 80 to get an extra firm ride. I really don't care. It's your money and your safety.

N_Jay

You don't read well do you?

Just a few posts up I said:

The issue is there is no "Best", "Perfect", "Optimal", tire pressure, just a "Recommended" (Minimum) pressure and a tire sidewall "Maximum" pressure.

Between these two, all the factors affected by tire pressure;
Ride Comfort,
Tire Wear,
Handing,
Impact Resistance,
Resistance to Hydroplaning,
Etc.,
are somewhere on the generally safe portion of their performance curve.

So if you are a; "I like the rules", "I listen to recommendations, "I don't want to think" kind of person, go with the sticker (or to be safe a couple of PSI above the sticker, unless you REALLY KNOW your gauge is perfect, AND you ALWAYS check the pressure before you ever lose a pound.)

If you are a car enthusiast, learn the factors and decide for yourself.

DaveW68

OK Mr Know-it-all....I can read just fine. I just don't subscribe to your "made to suit yourself" line of bullsh*t. I'll stick with the manufacturer recommended (not a minimum) PSI since I believe the MB engineers who designed the truck to be A LOT smarter than you are. You do whatever the hell you please with your own truck. Like I said before, it's your money and your safety. Your time would be better spent arguing with an AOC or Bernie supporter since they seem to be about the same level that you are.

Have a nice day.

kombifan

I see no reason to bring politics into this conversation.

Everyone has their viewpoint, and their opinions on what they should do.

It it is your vehicle, and you may do so with it as you please (as required by law of course)

That being said, tire psi is a very subjective subject.

Let’s keep it informative and civil guys.