Thank you Cooljjay ... I also use 35 lbs ...

Here is a clue to the physics I am referring to...
There is reason for calling for checking your tires when they are cold....
and never letting any air out if you check them with them hot... and hot they may be over the number listed on the side of the tire or the one on the fuel door...
but if you let it out at that time...
then as soon as it cools off it will be under inflated....
and cause way too much side wall flex heat to be generated...
and you will wind up with even higher psi than you found when checking it hot....

No problem! I always try to step in and help when I can....I suppose its because I am such a history/antique nut that I have really seen how things have changed over the years....from our food to our furniture...Its really sad when folks never stop and see that things change...sometimes for the better and more times for the worse...If they would bring back history in school and current history maybe things in America might change....until then the next generation will continue to wait out side the Apple store for the next new gizmo....

Apologies to the OP, but I'll keep this totally unintentional thread hijack going for one more post.

If you run with the recommended pressures, the tire will roll onto the sidewall during max effort cornering. Sidewalls aren't very grippy. I use a higher pressure to keep the tire cornering on the tread.

Real world example: Running the pressures on the placard, I can take the inside lane of exit 57B from I-580 at a maximum of 50 mph. Running 40/38, I can take that same turn at 56-57 mph. That is a significant improvement.

Also, if you look at the link below, what I'm running falls into the recommended range for competition. And I tend to drive like I think I'm Ayrton Senna.

http://www.tirerack.com/tires/tiretech/techpage.jsp?techid=58

the information I've gotten from tire dealers and drivers and my personal experience with cars and trucks and racing over the years...

the tire max pressure rating is really for use at the max weight the tire is spec'd for... I would only use that pressure if I had the max weight on the tire.

for instance... most road car tires have a 1300KG max load rating... so... that's a LOT of weight.

high pressure should be based on how much mass the tires are expected to support...

if the tires are on a coupe with little in the trunk, and you're in fit shape, and the fuel tank is full... I'd set the pressure to the specs on the fuel door... notice that the specs CHANGE based on what's in the vehicle, and what speed it is driving at. higher speeds equate to higher sidewall loads, and the need for higher pressure to prevent bead separation.
the rubber compound has little to do with the air pressure recommendation aside from ply compound and strength for weight/speed. it's all about the air supporting the vehicle. heavy loads, higher pressure... high speeds, higher pressure. speeds exceeding the tire's speed rating need to be accommodated by higher speed rated tires, and set to the correct pressure for that speed.

I'd never recommend anybody driving their car with average weight and average traffic/speeds EVER set their tire to the max pressure rating on the side of the tire.

the information I've gotten from tire dealers and drivers and my personal experience with cars and trucks and racing over the years...

the tire max pressure rating is really for use at the max weight the tire is spec'd for... I would only use that pressure if I had the max weight on the tire.

for instance... most road car tires have a 1300KG max load rating... so... that's a LOT of weight.

high pressure should be based on how much mass the tires are expected to support...

if the tires are on a coupe with little in the trunk, and you're in fit shape, and the fuel tank is full... I'd set the pressure to the specs on the fuel door... notice that the specs CHANGE based on what's in the vehicle, and what speed it is driving at. higher speeds equate to higher sidewall loads, and the need for higher pressure to prevent bead separation.
the rubber compound has little to do with the air pressure recommendation aside from ply compound and strength for weight/speed. it's all about the air supporting the vehicle. heavy loads, higher pressure... high speeds, higher pressure. speeds exceeding the tire's speed rating need to be accommodated by higher speed rated tires, and set to the correct pressure for that speed.

I'd never recommend anybody driving their car with average weight and average traffic/speeds EVER set their tire to the max pressure rating on the side of the tire.

What VS says.

The pressure recommendation in the filler flap is for the stock size tire (listed) on the stock size rim, with the weight of the vehicle loaded. Mercedes-Benz did a fair amount of testing with this combination and found that this pressure in this size tire gave a good balance between handling, ride, economy, safety, etc.

You can add air pressure if you wish, but you will change the dynamics of the vehicle. You might reduce rolling resistance and increase cornering response, at the cost of worse ride and possibly worse handling on rough pavement, you might change the rolling tire patch on the ground leading to more wear in the center of the tire, you might change the understeer-oversteer balance of the vehicle.

Inflating less than the automaker's recommendation is always bad, unless you're traversing a sand-dune or otherwise trying to lower the planing speed of the tire for off-road use.

The inflation pressure will not change with newer tires, but it can change with different size tires. Playing with tire pressures should be done with the knowledge of what you expect to gain and typically we do it on a track, find an SCCA weekend on a local track to do your testing once the tires are warm.

Some of you are acting like the sky is falling...
As someone who has actually added up the weight I was dealing with ... and dividing it by four... and looking at the recommended psi on the side of the tire....
The idea that the recommended psi could not change due to changes in tire manufacturing over a 35 year period ..... Really ? Those engineers had control over all the other stuff at the time they made the car... but they could not know what a replacement item like tires would be like 35 years later....
Are the fuel door recommendations different between my 240d and the heavier cars ?
They also had no way of knowing the difference between the weight of two average passengers 35 years ago in Germany.....and two Americans NOW....

Great thread!

I've always inflated my tires higher than the door placard recommendation. As stated in the linked article, lower inflation will cause increased wear and worse handling performance. The numbers in the placard are a compromise that the auto maker settled on, and they considered the ride quality (noise, bump feel, steering feel, brake feel) as well as fuel consumption and handling performance. As pointed out, inside the fuel door is another sticker that has "normal" inflation pressures for front and rear and "high speed or increased load" pressures. I tend toward the increased load pressures, usually even a couple pounds more.

I would submit that tire wear had a lower weighting than safety and ride quality.

Note that the linked article relied on information from the tire makers, who like to sell tires. Do you think they would like our tires to wear out faster?

Last year I researched this, and read several forums devoted to hyper-miling (maximizing fuel economy). Many contributors ran their tires at max sidewall pressure and reported that tire wear decreased dramatically. Some reported that tire life nearly doubled.

In my opinion, the biggest argument against over-inflating is the reduction in the tire contact patch which negatively affects braking (safety). I see this as balancing economy (tire wear and fuel economy) vs. safety.

... best way to determine proper tire pressure for a given vehicle is the chalk method. Chalk a line across the tread and drive. The chalk should be worn off evenly for best overall tread contact and tire life. It shouldnt take but a couple rotations of the tire wear off the chalk... and it needs to be on a flat surface such as concrete.

Just my 2 cents... as proper pressure changes with every vehicle (weight, etc..), tire compound, etc..etc..

Over inflation will prematurely wear the center of the tread width... under inflation will wear the outer portions of the tread prematurely.

The recommended tire pressure by the manufacturer is for the tires that came on the car from the factory. If you are not using the OEM rubber.. the sticker doesnt exactly apply.

Thanks for making this a standalone thread John. I also appreciate your input on the matter.

Yes Greg, the pressures in your 240D filler door are different than the ones in a 300D door. That sticker has a part number on it which is specific to the vehicle.

I don't think human size has changed in only 30 years. 30,000 years maybe. An average man is 180 pounds. An extra 100 pounds isn't going to make a huge difference. That probably the delta between full and empty fuel tank.

There are normal load pressures, high load high speed pressures, and also warm tire pressure (says add 4 psi) on my sticker.

I think you guys are thinking like your 240s and 300s are race cars... They were built to be comfortable, economical, long lasting, safe, and smooth. They were meant to have soft tires that gripped strongly and cushioned road hazards. The speed limit in the US was 55 on highways. Yes, they had the Autobahn in Germany, and that's why they gave you the high speed rating so you could inflate to that if you were going to do 100mph flat out. If you drive at 75 highway in the boonies, fine, inflate to the high speed pressures. 40 pounds is just too much though.

My 190D calls for 26psi front and 29 rear.

Hypermilers often want to squeeze whatever highest mpg they can out of their car come hell or high water. They will coast in neutral (dangerous), coast with the clutch depressed (harsh on the TO bearing), inflate their tires to the max listed on the sidewall (sacrificing grip and putting lots of stress on the tire), and some will even turn their engine off while coasting and while at a red light (do I even need to say why that's dangerous???). Forgive me if I don't take auto advice from a pure hypermiler.

As for tire compounds, they haven't the slightest effect on pressures. Grip, hell yeah they do. LRR tires are hard. Snow tires are soft. They still use the manufacturer specific pressures that were glued onto your automobile. They even put it in the fuel door because they want you to make sure they are at that pressure every single time you fill your gas tank.

It's interesting how in regards to the stock cooling system any aftermarket alterations are poo pooed, yet the manufacturer specific pressures that were given as much thought if not more as the cooling system are tossed aside and labeled obsolete due to age (as if a metal engine fan is not). I just see hypocricy in full force here, that's all.

I used to work at an old time alignment shop... He would measure the tire temp, align the car, then go drive his course and he always started with 30 front and 25 rear. After the drive he'd measure tire temp across the tread and adjust alignment to temp, then adjust pressure to overall temp.
Cars came to his shop from hundreds of miles for his voodoo.
He usually ended up with a hot pressure of 38 to 44 which is a cold of around 34.
I regret never getting training from him. He died stupidly a few years ago...

Tire design absolutely affects tire pressure recommendations. .. but the overall starting point is for grip and handling. Changing pressure is a trial and error thing that requires measuring data.
Just feeling the car how it drives isn't enough.

Think about it like this...

The rubber isn't holding up the car... it's the air.

The rubber is the contact point for the road. Adhesion and longevity are the rubbers job.
Air holds up the car.
A skinny tire needs more air pressure to do an equal job to a wider tire...

Ever ridden a bike?
Bmx tires with a 3" contact patch get around 25 psi...
Highway bike tires with a 3/4" patch need 100... your body is the same weight...

I do 40 all around on the 300d. The max psi on the sidewall says 51psi. At 40psi there is less sidewall flex and less heat buildup in the tire. The 33psi written on gas door is a compromise between ride comford and tire life with tire tech from 30 years ago when the national speed limit was 55mph. Who still drives 55mph? Modern tires at modern speeds work much better at higher pressure. MB recomends up to 41psi for high speed driving. its right there at the bottom of the sticker so 40psi isnt unprecedented.
http://www.benzworld.org/forums/atta...1245843239.jpg

I also mount and balance my own tires (no wheel weights) but thats another story.

My C2500 truck with E range tires can be inflated to 96 psi... if I needed to haul 10K on them... my truck is around 6K normally so I keep around 60 psi in there...

When towing I bump it up to 90... on the rear.

OK..... I am going to type out the FSM page concerning ' Tire inflation pressure' ...
Two things to look closely at....
1..the emphasis on bad things which come from underinflation
2.. in the ' over inflation warning'... is for OVER the MAX load value..... no one has been suggesting OVER the max load psi suggested on the tire.

40.5-106/1 1987 Service Manual Chassis and Body Series 123 , volume 1

"""...A wrong inflation pressure, particularly when the air pressure is too low, will influence the driving characteristics and the life of the tires depending on extent of deviation from specified value, and will also lead to an additional , higher fuel consumption.

If the tire inflation pressure is too low, flexing and thereby excessive heating will increase. The understructure of such a tire will lose its compactness. The results: Tread and belt will come loose. Depending on size of reduced inflation pressure and driven speeds, the life of the tire will become shorter, while even short-term ' inflation sins" may lead to permanent damage.

On the other hand, a tire inflation pressure which is essentially too high ( higher than the values named for fast driving or for max. loads ) incorporates the disadvantages of a high loss in comfort, while the smaller tire road contact area results in worsening of driving characteristics and on a wet road also in a higher trend toward aquaplaning. """

Two pages later is a full page chart... which is in BAR listings.....and is hard to make readable without actually using a chart to represent it...

It has the cars listed listed with front and rear tire pressures for Summer tires up to 160 km/h .... above 160 km/h ... and Winter tires up to 160 km/h. That is about 100 mph. In each of the boxes in the chart the pressures ( in BAR) are PER AXLE.... how many of you put different psi into the front and rear tires ? Is that listed on the fuel tank door ?

So for the model 123... which includes 123 with these numbers after the decimal point..
02,03,04,06,1,22,24..... the front summer tire , less than 100 mph, front axle is 2 bar..30 lbs.... the rear axle 2.2 bar.... 33 psi.

Sedans ' long' 123.028,125,132 is 2.2 front summer tire, less than 100 mph, front axle is 2.2 bar.... 33 psi..... rear axle 2.6 bar.... 39psi.

T Sedans ( wagons ) with higher add load permissible rear axle load 1300 kg ... front 2.3 bar ...34.5 psi..... rear 3.2 bar..... 48 psi..... ( not a typo )...


The chart goes on to list all sorts of things like Ambulances, Funeral coaches, Ambulance long, Funeral coach long, special sedans with higher permissible rear axle load 1200 km ( I would guess this is a sedan with the SLS rear auto adjustable ride height .. but I am not sure of that )....

The highest psi listed.. in bar.... is for a funeral coach with winter tires up to 100 mph at 3.5 bar... 52.5 PSI ... !!!!!

These were recommended in the days when road speeds averaged much less than most of us drive... and road were not as good then.... most of the US interstate highways were concrete slab with asphalt seems every 10 feet or so... that will drive you crazy going very far....

I will be using 35 psi for front AND rear axles on my cars... notice the tire recommendations said high load OR high speeds.... I know most of us drive 70-75 regularly out on the highway.....