mbnj

What year wagon? Amazing, I have a 2016 4matic wagon completely different for both front and rear.
I fill to the spec inside the door frame .

thenew3

2013 wagon with the AMG sport pkg 18" rims. The sticker in the gas filler flap says 35/41 f/r normal load and 35/48 f/r max load. driver door sticker says 35/48 f/r

jahquan3

Wow that max load seems to be the largest difference I've seen between front and rear PSI. I guess they're taking into account all the stuff that could be packed into a wagon.

thenew3

Maybe. I have hauled over 1300 pounds of wood and tile in the rear of my wagon a # of times (home remodel). It's impressive how much construction material can fit in the back of the wagon.

I did pump up the tires to 48 psi when I did that.

mbnj

Right. My frame says 35/48 as well. My previous wagons have shown as much as 6 lb differences but 13 lbs really caught my attention. I'm running 245/40/18 also with the AMG's.

degeja

Interesting; I've had to lower TP by several psi to keep tires from wearing down the middle when one person mostly in car

jadatis

I edited my Post#22.
answered also to post later of thenew3 about the different advices for same car.
But now read also about fuel saving, and ofcource it is also done for that.
Once read in a research of official institute for trucks that roling-resistence makes about 35% of total fuel for a 18000kg/40000lbs truck, with ofcource more wheels and more deflected then a normal car, but still estimate for a normal car, roling restistence to be about 20 to 25% of total fuel use.

But this fuel saving could also be an excuse to not say the real reason , and that is savety , otherwise they also would give front high advice , and they only do it for rear.

Diesel #2

I run 33 ad 35. During hot days it gets up to 40 and 42. But my tires are low profile and low air pressure means banging up your wheels.

jadatis

@ Diesel#2
I looked in a tabel I made about pressure rising for temperature.
Your 33/35 psi cold filled at 55degr F would get on a hot day cold 36.3/38.4 psi at 90 degr F, and heating up by driving at about 55m/h, I estimate the tire inside to get about 40 degr hotter so 130 degr F, at wich the pressures rise to 40/42.3 psi .
And that is yust the pressure you give on a hot day.
So probably your pressure-settings are right.

Normal I once determined is when outside tire temp = 65 degr F , the temp inside the tire rises, when you drive about that 55m/h , to 110 degr F.

Goal of pressure for load and speed, is to give rubber of tire , at no spot of it, a temperature at wich it hardens and crackes in next bendings/deflections.

At higher outside temp, higher cold pressure , gives lesser deflection so heatproduction a cycle, but also cooling down of tire is worse because of the smaller temperature differences. Most likely this heating up and cooling down stays in line between an outside tire temp ( ambiënt temp) of 10 degr F to 100 degr F.
So for savety you dont need to highen up the pressure to advice when its colder then 65 degr F , but you must certainly dont lett of air to adviced when the outside temp is above 65 degr F.

mellonc

Jadatis

what is your thought on going up to the mountains to ski and stuff.... I noticed from sea level to 7000 ft high, the temperature changed from 60 degrees to 25 degrees. In that time, I expected the tire pressure to drop something like 6 to 8 psi but it didn't do that. It perhaps dropped about 3 to 4 psi. Is that due to thinner air???

jadatis

looked it back with google and estimate that at 7000 ft the pressure is about 70% of that on sea level so about 10,4 psi instead of 14.7 psi average.
The pressure inside the tire is measured overpressure above the ambiént outside tire pressure .
so I asume you determined the 6 yo 8 psi higher for the temperature rising right , the real pressure in tire becomes that 6 to 8 lower, but the outside tire pressure has dropped also about 4.3 psi ( 14.7-10.4).
This gives aproximately that 4 psi higher measured pressure in tire at 7000 ft.

Because you estimated the pressure-dropp and did not give the tirepressure , can explane the little difference between my calculation and what you experiënced. So yust for the explanation of the principle.

I found with google on Dutch Wicopedia, that at 5000m/15000 ft the pressure is 50% of that on sealevel.
and the pressure chanches exponentially with hight , wich means that at 10000m/30000ft the pressure will be 50% of 50% = 25% of that on sealevel. Rest is estimation in my story . Did not do administrative reasearch on the hight thing yet, mayby a new part for my tirepressure -workshop.

Diesel #2

Well said, that is precisely correct. Cold start around 33 / 35, after 20 mins of driving in the city on a 82 degree day gets up to 36 / 38. And at highway speeds it gets up to 40 / 42.

jadatis

On hihgway the pressure rises, because of the higher speed. The tire makes more cycles a second so more heatproduction a second.
And that in combination with higher outside tire temperature , it can rise to that higher pressure , but still save because the rubber still dont get above a sertain temperature at wich it hardens and damages in next bendings /deflections.
This because though the outside tire temp is higher, the pressure rises more so lesser deflection so lesser heatproduction a cycle.

I will give the picture I made about pressure for temperature, so you can play with it yourselfes.

basic idea is that at 65 degr F cold pressure is fat printed and the advice the car maker gives . Then it rises to aboutu 110 degr F when driving constant at about 55 m/h , then look in the tabel how high the pressure will get.
The temp in tire warm then is 45 degr hotter then outside tire temp.

But when its hotter outside the the temp in tire risses less to give the same temperature of the hottest parts of rubber of tire.
The faster you drive the hotter inside tire temp.
I estimate that at 65 degr F outside tire temp the inside tire temp rises when riding 160km/99m/h constant to about 140 degr F .
At that temp inside tire the rubber stays below its temp where it hardens and damages.

tresean1

Hey guys,

I had something weird happen this morning on the way to work: my tire pressure light came on (first time I have ever seen this). Then I saw the 4 tire pressures bouncing all over the place: front left went from 36 to 33; front right went from 34 to 35; rear left went from 35 to 34; rear right went down to 32 then back up to 33.

What to make of this? Is this normal?
Does the Tire Pressure light come on when the pressure goes below a certain set point. or does it just come on when the 4 pressures are unequal?

Is this something to be concerned about?
I have a 2014 E350 4MATIC with 18" AMG wheels.

jahquan3

Once you set your tire pressures to a desired number. The system will flag if it drops below 5-6 PSI of that set figure. So you can have all of your tires set to a different PSI and lock it in and they system won't flag it. It's more so to let you know that your PSI has now gone relatively far from what you told it was your desired PSI.

I've never seen it bounce around weirdly as you've described. I have had a few slow leaks and a couple blow outs. So I've seen that alert you got before.

thefisch

Yes, you need to make sure you set the reference values when the readings show the correct pressures. The light goes on and center cluster goes red when a tire reports a loss of 25% from the set value. You get a soft warning when turning off key (like other reminders) when value is 10% below reference value.

It is normal for values to change when driving but they normally move 1 psi at a time as they heat up, not 3. Keep an eye and that and check the pressure manually to make sure the car readings are accurate. My car is always within 0.5 psi of my gauge reading.

tresean1

Thanks guys,
I actually stopped by the dealership yesterday around the same time jahquan replied and they told me the same thing. They actually filled all four tires up to 37psi using the tire gauge, although the vehicle's TPSM read between 37-39 for each tire (go figure! lol). So we reset the TPSM using those benchmarks; they said if any one of the tires falls below 10% of those pressures, then it will trip the yellow warning light. They said it's really nothing to be concerned with unless the pressure goes way below 30psi.

thefisch

It's interesting that on MB they let you set the reference value while my others cars with TPMS (Honda and Toyota) do not let you set a reference value. I believe they go off the door placard and the warning trips when any tire is near or at a 25% loss from the door placard. Interesting that the TREAD act final rule specifies a warning at a 25% or greater loss from door placard from what I have read. So if MB lets you set your own reference value, in theory you could set the reference value so low that the warning may not comply with the law (i.e. 75% or less of door placard). I wonder if the Germans have figured out a fail-safe for that too - or maybe I am not good at reading law text.

KEY08

My daughter's 2015 Honda Fit allows you to reset and restart the TPM sensors where you have filled the tires up to and works off of that value. There is a reset button just below the dash on the left side. How old are the Hondas you are referencing, Fisch?

thefisch

Mine is a 2010. There is no reset button or ability to view values. Manual says nothing and I looked under dash and saw no buttons. Dealer told me it is preprogrammed. Probably different on newer models.

cetialpha5

I suppose that makes sense if you're planning on driving 100 mph which has a different set of values. The Honda probably doesn't have that option. So if you've got the tires pumped up for high speed driving, then it'd make sense that you'd want to know if they go a little low even if that pressure would be fine for normal driving. They could have it hard coded to display a warning once it reaches a certain number so maybe you can't set it too low. But who knows, I guess someone could experiment if they were really interested.

tresean1

I guess they feel that, in general, Mercedes and BMW drivers are generally more attention detailed and performance-oriented than "normal" drivers. They are probably looking at it from the standpoint of 'what's the chances of a Mercedes driver setting the reference point at a low or unsafe level in the first place?'...Versus, let's say, a Honda or Chevy driver carelessly messing around with the TPMS.