Hundens

We have a MY19 E450 Wagon with R26 19" Ten Spoke Wheels and R02 All Season Tires (Pirelli Cinturato P-7). The BW had the tires filled at Costco today because we were getting a low tire reading on one tire and because Costco uses nitrogen for the fill. The TPMS via MBConnect is now showing 44 in front and 50 in the rear. I presume the Tech was going for the pressures on the label on the driver's door jam of 42 and 48 psi. I also presume that the pump at Costco can accurately generate a tire pressure on demand (there is a gauge on the hose that the tech sets for the desired pressure). So first question. How accurate is the TPMS versus reality. I use a hand held electronic tire pressure monitor to check for myself and I find the TPMS to be 2 pounds higher.

Now for the interesting part. If you open the fuel filler door you find a label that says the normal tire pressure is 35 PSI all the way around, and the max load pressures are 38 front and 44 rear, in both cases for the 19" tire.

So, second question, WTF is going on. That's three sets of pressures on the same vehicle, forgetting about the accuracy of at the TPMS.

HELP.

ua549

3 sets of pressures is normal - normal, full load, high speed. IIRC high speed is +4 over normal or full load whichever your base is.

JTK44

I too have a MY 2019 E450 with Pirelli Cinturato RF tires with 18" wheels - standard on the Luxury Styling.

The gas filler for 18" wheels recommends 33/33 front /rear and 35/40 for maximum load.

For 19" wheels the recommendation is 33/33 front/rear and 35/42 for maximum load.

Maybe only 1 out of 100 trips do I have maximum load so I use the 33/33 recommendations.

However, I have also keep pressure slightly higher and use 35/35. I find the handling is slightly sharper, mileage slightly better and ride the same as 33/33.

I maintain pressure with a good air compressor. I also find a drop of 10 degrees and the tire pressure drops 2 to 3 lbs.

To check my tire pressure I use a an electronic tire pressure gauge. see: https://www.accutiregauge.com/digita...al-tire-gauge/

I have checked the pressure on my tire gauge against the TPS and they are the same pressure.

I have cross checked the gauge against the TPS in my second car, a 2018 Ford Edge Sport. The gauge is always within 1 lbs. of the TPS. So I must assume my gauge is accurate and since the TPS always shows the same pressure as my gauge I must assume the TPS in my Mercedes is also accurate.

So bottom line is I have no idea why Costco put 44/50 lbs in your tires other than that might be the maximum rating of the tires. If it where my car I would reduce the pressure to 35/35 with a good tire gauge and then cross check it against your TPS.

Hope this helps.

2012 merc amg

I have that happen a lot. It seems like garages always put too much air in the tires. First thing I do when I get the car home is let the air down to 35 all the way around.

Hundens

Where in the literature do you find a "high speed" pressure?

joshg1001

Use the label on the fuel cap. That one is specifically calculated per your car's specs.

ua549

It is stated on the fuel filler door as well as on the left side door jamb label.

JTK44

The suggested tire pressure on the door jam is often different than on the fuel filler door.

I always use the tire pressure on the fuel filler door.

The "high speed" tire pressure does not apply to the US unless you intend to drive for an extended period of time above 100 mph.

dudley07726

I had the same thing done at Costco. I had them install Michelin Pilot Sport A/S 3+ on my Audi. They were following the label on the door jam which showed H rated tires and ratings of 38 F / 41 F cold.
I told him that label showed maximum load pressure for H rates tires. I showed him in the book that the correct pressure should be 36 f / 33 R cold for high performance Y rated tires.
They have no clue.
BTY. While their price was good, I wasn't thrilled with how they treated the rims. For my 18 E400, I got rid of the lousy Goodyear RFT's and replaced them with the same tire on my Audi. This time, I ordered from Tire Rack and went to a local independent who did a much better job of installing the tires on the rims. Left them immaculate.

ua549

The pressures are the same, just stated differently. The difference is in the fine print labeling. IIRC the fuel filler has 39 +4 for high speed and the door jamb says 43.

I use the high speed settings on my other cars when I'm cruising the Interstate and the ambient temperatures are over 100° F. This happens mostly when I'm traveling in TX, OK, NV and AZ. My W213 never leaves the city.

DFWdude

I just checked mine just yesterday...

JTK44

My TPS gives pressure and temperature.

As I drive I see both the pressure and temperature increase.

.

The_Judge

Don't trust the tire pressure monitor to give you accurate readings. They're generally in the ballpark, and the system will warn you if your pressure drops below a safe level. Get a good tire gauge and use it when you check the tire pressures — for one thing it's the same gauge for all tires, not four different ones. You'll likely find up to a 2 or 3 PSI difference between the reading from the "real" gauge and the readout on your dash.

rraisley

I have a good/accurate tire gauge. I check the pressure with it, then compare what the car's monitor says, and it's always with 1 psi, often right on. Same with my Kia. I think the tire pressure sensors are pretty accurate these days.

DFWdude

Yep, I agree. I have a good quality dial-type gauge, and it always reads within 1psi of the TPMS values... most often exactly the same.

JTK44

+1

see post #3