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  #1  
Old 05-04-2003, 07:10 AM
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Location: Melbourne, Australia
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Question correct tyre pressure for comfortable ride

I drive a Euro C180 1994 model and today it felt quite hard like the tyres are over pumped. Before the last service, I had it very low like 28psi and the ride was very soft and smooth. At the service I was told it required 36psi which I thought was far too high. What do you guys think, is 30psi just right? The car is only 1350kg kerb weight and thats quite light compared with my Camry which is 1830kg with a 2.2ltr engine compared with the 1799cc of the C180. Tyres are 195 standard 15inch steel rims.
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  #2  
Old 05-04-2003, 01:15 PM
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Don't know about other countries models, but US models have a placard in the door jam or behind the glove box or center bin cover that contains the manufacturer's recommended tire pressure, and they are usually fairly low to give ride comfort, but may contain additional recommendations for high speed driving. Each country probably had their own homologation requirements for recommended pressure and how it is displayed.

Alternatively, you could check your owner's manual to see if recommended tire pressures are contained there.

As a final sanity check, find the Max load @ max pressure placarded on the tire and compare it to the normal weight of the car and use common sense as to what pressure is sufficient to carry the actual load that the tires see in service.

Chances are that 28 psi is adequate, but very high ambient temperatures or high speed driving would call for higher pressures to maintain a reasonable safety margin. Also, lower pressures will reduce steering response.

One should never go below OEM recommended pressures but higher cold pressures - up to the maximum placarded on the tire will yield better handling, lower fuel consumption, and increase tire life but the ride will be harsher. The recommended pressures for the OEM 185/65VR-15 tires on my 190E 2.6 are 23/29, if I recall correctly, and the car drove horribly. To mitigate the understeer and give decent steering response I ran them at 42/36. The were placarded at 44 psi max cold pressure. I run 36/32 on the currently installed 205/55ZR-15 Dunlop Sport 8000s on 6.5" 300E wheels.

Duke
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  #3  
Old 05-04-2003, 01:38 PM
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Typically on U.S. models, air pressure for tires is found under the gas filler door.

The presumption is that all tire pressure measurements are done when the tires are cool, such as first thing in the morning before driving OR less than one mile of driving.

This chart also shows additions to the basic recommended pressure for additional passengers and luggage. Further, it points out adding +4 lbs for sustained high speed driving (above 100 MPH).

Never exceed the tire manufacturer's sidewall max inflation recommendation.

I NEVER rely on service station pressure gauges. I have started using electronic air pressure gauge which I like, especially because it has the "Zeroing" feature. This site sells gauges as does your local MBZ dealer.

For every 10 degrees of ambient temperature your tires will change approximately 1 psi. If you haven't checked your pressures in say three months, you might be astonished what they are set at.

Generally speaking, the higher the tire pressure over the basic recommended pressures, the greater tire life.

Keep us posted,

Haasman
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  #4  
Old 05-04-2003, 02:57 PM
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Go with the OEM specified pressures.
Remember:

Over inflated = centre tread wear
Under inflated = outside tread wear.
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  #5  
Old 05-05-2003, 09:27 AM
LarryBible
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NEGATIVE!!!!!!!!!

In spite of the outdated illustrations seen in many places, on a RADIAL tire overinflation does NOT lead to center tread wear unless it is EXTREME overinflation. Those illustrations are holdovers from bias ply days. On radials, underinflation is MUCH more commonly the cause of center tread wear, especially on rear tires.

The placard is the MINIMUM cold inflation pressure.

Good luck,
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  #6  
Old 05-05-2003, 12:53 PM
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Larry is right on the money. Tire pressure, like wheel alignment is a chassis tuning parameter. For the US, DB tends to recommend minimum pressures for ride comfort, but better handling, reduced fuel consumption and tire wear can be obtained by running pressures in the thrities, and since most Mercs are front heavy with just one or two people aboard, they will respond favorably to biasing the front pressure higher than the rear. This will improve steering response and reduce understeer.

Like most rear drive cars, the tires tend to wear faster on the shoulders at the front and faster in the center at the rear, so regular tire rotations - say at 7500 mile intervals - will tend to even out the wear.

Duke
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  #7  
Old 05-05-2003, 01:35 PM
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Well, I am not so sure where you guys get your information from.
On my fuel filler door it syas:

" FOR MAXIMUM LOADS " !!!!!!
Front: 28 psi Rear 32 psi COLD.

" FOR SUSTAINED HIGH SPEEDS ( over 100 mph ) increase cold pressure + 4 psi.

When in doubt, run slightly higher, rather than too low a pressure.
P.S. If you are " seeking the truth ", I would suggest asking the reals experts at The Tire Rack.
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  #8  
Old 05-05-2003, 02:18 PM
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Quote:

P.S. If you are " seeking the truth ", I would suggest asking the reals experts at The Tire Rack.
They are hardly "experts" at determining tire pressures for all cars, drivers, and driving conditions.

All tires are placarded for their maximum load capacity at maximum COLD inflation pressure, but since OEM tires are always selected to have considerable load margin, the maximum cold pressure placarded on the sidewall is rarely required.

Mercedes' tire pressure recommendations are very conservative, and more rear pressure is specified because at gross weight with rear passengers and the trunk loaded, the car will be rear heavy, but under typical load conditions of one or two people the car is front heavy. This is why running positive front tire pressure bias will improve handling, but Mercedes recommendations are conservative to ensure that the car will understeer excessively.

As I said, tire pressure is a tuning parameter and the only basic requirement is to maintain sufficient pressure to handle speed and load conditions, but higher tire pressures are optional as are different pressure biases to tune the car's handling to individual preference.

As a point of reference most lower speed rated tires are placarded at 35 or 36 psi maximum cold pressure. V and above speed rated tires are usually placarded at 44 psi, and some of the newest Z-rated tires are placarded at 51 psi, maximum cold pressure.

Duke
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  #9  
Old 05-05-2003, 02:28 PM
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Location: Metrowest, Mass
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Are these weights right??

Is anyone else surprised by the weights quoted in the original post? The 1350kg (2970 lb) C seems quite light compared to the 1565kg (3450 lb) of a current c320, and the 1830kg Camry (a whopping 4026 lb!) seems awfully heavy!
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  #10  
Old 05-05-2003, 02:43 PM
LarryBible
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manny,

If I read your post correctly, you are going the long way around in confirming my statement that the placard indicates "Minimum Pressures."

If you run the pressures indicated on the fuel door, in my experience that is almost a GUARANTEE for center treadwear, particularly on the right rear (don't ask me why the right rear will do this more than the left rear because I don't know.) You need to add a few pounds to the rear loaded number.

That's been my experience from almost a million miles of keeping tires on MB's.

Have a great day,
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  #11  
Old 05-05-2003, 03:01 PM
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Join Date: Sep 2002
Location: NE
Posts: 133
DIY tip

not that anyone will find this helpful or insightful but I was just told how to build free jack stands on a different post and thought I'd share the love and contribute something trivial here: my stand up bicycle pump works on the car and motorcycle too. no need to leave the house and drive to the gas station to pay a quarter to unravel a hose that doesn't seem to have any air...

cdt
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