I got the correct tire pressure warning today, so I checked the door and gas cap and it seems like it should be 38psi in all the tires. Is that right, or has anybody found a better setup?

Great question that I have been dealing with lately since my 2012 E 350 cabriolet went in for its 1 year check up... It appears that there are 2 stickers, one on the drivers door and one on the fuel door. The one on the drivers door is correct and customized for the vehicle and outfitted tire package. Since I selected the AMG wheels, my tires require 36 front and 46 rear pressures. My rear tires have been under inflated by 10 psi for almost a year..luckily I only drive the car on perfect weather days so hopefully not a big deal. They are now at 46 and the tire pressure monitoring system has been reset for 36/46 psi.

The correct tire pressure for an 2012 E550 coupe with AMG wheels is 35 PSI cold all the way around. 46 sounds a little high, but trust the sticker. Just make sure you're not using the MAX tire pressure, but the recommended cold pressure.

Our vehicle was ordered with the summer performance tires and the recommended cold pressure is 36 front/46 rear.

There's no such thing as "correct" pressure. The reason they (MB) want E550s to use 36/46 is to make sure there's plenty of understeer left for those who could not handle all that torque in a light car. There is a high possibility of someone "loosing" the car with fron/rear pressure close or the same - it will induce oversteer which is much harder to handle.

That justification would make more sense if the higher pressure was in the front, or if MB did that across the board. For example, the factory recommendation for my '12 550 is 35 PSI all the way around. MB dials in understeer with staggered tires (235 front, 255 rear).

Over-pressurizing a tire causes it to "round" and reduce the contact patch. Doing this to a rear results in oversteer.

More likely, the recommendation is a result of the specific tire design. My '11 had Michelin Primacy MXM4 and the recommended pressures were 33 PSI front, 35 PSI rear. The Pirelli PZero Nero M&S have 35 PSI recommended all around. And I'm still experimenting with pressures on my Continentals to find the right settings.

What Summer tires does MB install? I'd like to do some research and figure out what the difference between those and the Pirellis is, and why the huge difference in recommended pressure. I suspect it may be related to tread width, but need the specs for calculation.

This is a constantly recurring question with Mercedes drivers, because of the existence of the two differing placards on the same vehicle. Mercedes wording as stated in the Owners Manual essentially sets upper and lower tire pressure limits - a range of safe and acceptable pressures - not a single specific pressure to which the car's tires must at all times be inflated.

The placard on the driver's side B pillar represents the maximum tire pressures for the car's maximum load (e.g. 5 people on board and trunk full of luggage) and maximum speed for which the vehicle is designed. One should not exceed these tire pressures.

The placard on the gas filler cap represents the recommended pressures for more typical or "normal" day-to-day carrying load weights and operating speeds. One should not allow pressures to drop below these values.

These stated placard values vary with the specific vehicle model and with the model of tires with which it was equipped when it left the factory.

If your car is equipped with the stiffer sport suspension, I cannot imagine operating it day-to-day (aka suffering) needlessly with maximum inflation values. But that's just me. I get all the steering and overall handling response I need using the minimum recommended values stated on the gas filler cap placard. Your kidneys may be more absorbant than mine.

That might be the answer: ours came with SP Sport Maxx GT.

Of the three (2011 w/all season, 2012 w/all season, Dueclaws' Summer Performance), the treadwidth correllates with pressure, but not in a linear fashion and I doubt it's causitive. I've been trying to find guidance on calculating proper tire pressures, but most of what I've found is limited to industrial and farm vehicles.

I found one article that recommended 90% of mfg. limit, but that was on a hypermiling site and would result in severe center tire wear and reduced contact patch.

I did find this article (http://www.turnfast.com/tech_handling/handling_pressure) which provides advice on finding the right pressure but relies on having a track handy and pulling lots of laps - not exactly appropriate for street tires.

I think you guys are making too much of this. The tire pressure recommendations are based on weight loading. The 36/36 recommendation on a 2012 550 coupe is for normal loading of the car. The 38/46 is for fully loaded. Now think about this. Where could you posibly put the extra weight? Answer mostly in the rear of the car thus the much higher rear tire pressures.

The discussion about tire pressure and understeer or oversteer is not germane

BTW whether added or reduced tire pressure causes understeer or oversteer is tire specific. For high sidewall height tires more tire pressure may result in better traction in corners due to the reduced sidewall roll. In rubber band tires the sidewall is so stiff they really don't need any sidewall help and the added tires pressure just makes the bottom of the tire round and results in less traction.

Actually, we're not discussing max vs. recommended. Dueclaws has the Summer performance package and the recommended pressure was 36/46; the Pirelli All-Season package recommends 36/36; the Michelin All-Season package specs 33/35. Different tires, different pressures on the same car.

Max pressure is easy since it's on the label stuck behind the gas door. What we're trying to figure out is a methodolgy for determining proper (ideal) pressure on a non-factory setup.

Given:
A = actual tire pressure
X = ideal (or recommended?) pressure
Y = Min. usable pressure
Z = Max. usable pressure
and Y < X < Z

For any tire in a given application, pressure below Y would be critical as a result of an exaggerated concave shape of the contact patch; pressure above Z would be critical as a result of an exaggerated convex shape of the contact patch; any pressure in-between Y and Z would be usable and as A approaches X, the contact patch approaches linearity or flatness and results in best-possible tire performance.

The question becomes:
Given a specific load and specific tire (size, model, etc.), how do you calculate X, Y, and Z? This is a non-trivial question as the same tire, same size on two different car models (or same car and tire size but different tire models), even from the same manufacturer, will have different recommended pressures. Auto manufacturers have a way of determining the proper pressue, but what is it? Experimentation may be the answer, but it's not practical for consumers. I'd like to find a mathematical way of (at least) estimating.

I have a personal interest in this as I'm running non-factory size and rubber. The simple math says that as the contact patch grows, pressure would be reduced to support the same load. But that assumes all other factors are constant, and we know they aren't.

Looks like I stirred things up...I agree with everyone; the sticker on the driver's side door is only regarding what the pressures should be for carrying a maximum load. The sticker on the fuel cap door are for "normal" driving--perfect for my situation. What got me confused was the MB dealer increased my tire pressures to the level of maximum load and certainly not applicable for me so I will change my pressures back to 35 front and 36 rear as indicated on the fuel cap door (Pirelli PZERO R-18 All Weather). Now I just need to figure out how to reprogram the tire monitoring system....
Good discussion.

Your question is ill posed. What is it you are trying to acccomplish, track day min lap times, straight line performance, good ride with reasonable tire wear etc.?

The only one of the above which concerns OEM's is the last. I would also point out from a lot of racing experience that any track setup is usually a poor setup for the street.

Exactly! Regardless of the target result, how do you calculate it? Do I need to wear out 5 sets of tires experimenting? Or is there a way to calculate the ideal pressure mathematically for one or more of the purposes listed above?

The long and short of it is no. Depending on where the weight transfer goes in whatever you are doing the numbers change and change a lot.

People use different tire pressures depending on the track, temperature surface conditions etc. There is no formula that I know of that encompasses all that. In road racing and circle track racing it is not uncommon to have a different tire pressure in each tire depending on the turns in the particular track. They even change pressures depending on the driver preference which is different for different drivers.

In drag racing some people tend to use such low tire pressurize in the rear that unless they go straight the sidewalls roll under and it makes the car uncontrollable unless you slow down a lot.

All this stuff is empirically derived by trial and error. OEM's tend to compute tire pressures based on weight loading and normal public road driving as a starting point. They then hit the test track like everyone else to refine their selection to accommodate ride quality,wet condition stability, moderate cornering etc.

I would suggest you contact the manufacturer of your aftermarket tires to see if they have done any testing relevant to your intended use. I know in the racing world the manufacturers give a starting point for tire pressures but from there on its trial and error and as you mentioned wearing out tires.

Actually higher pressure in the back will create more oversteer.

You could do what we do on a race track. Chalk the tread over to the sidewall. Drive the car as you normally would. Look to see where the chalk Is worn off. If it is too much worn onto the sidewall add more air, if not enough reduce air.

Thanks! That's an excellent suggestion - simple and effective.

With the availability of inexpensive temperature measuring equipment (under 30 bucks at places like harbor frieght) you can do what professional race teams have done forever i.e. measure the temperature across the tread. This is especially useful to set the corner presures on road or circular race tracks.