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  #1  
Old 02-02-2007, 11:03 AM
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Tire Pressure 420 SEL

I bought a new set of ContiProContacts and am confused about correct tire pressure. These are "all season" grand touring tires. The MB owners manual lists pressures for summer and winter tires (and no pressures all season). Even more confusing is the table that lists different pressure for front vs. back tires. Add normal vs. maximum load changes and the pressures change again. My mind is spinning. Everyone I have spoken to (Tire Rack and Continental USA) offers different answers. Anybody out there able to clarify this issue? Thank you. P.S. The tire size = MB factory spec.

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  #2  
Old 02-02-2007, 03:48 PM
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I knew I would have to explain this eventually so, here goes ....

I don't know if you remember the quote about the nine scariest words you can ever hear and if you don't here they are "I'm from the government and I'm here to help". If you do remember the quote then you are 1 step ahead of the game.

I am going to explain this in two time periods the first time period is "Pre-ERO" and the second one is "Post-ERO". The ERO part is "Explorer roll over" which is when people in the southern states driving Ford Explorers were having tire blow outs from running to little air pressure in their tires, thus overloading and over heating them, and flipping the trucks over.

so, Pre-ERO
German vehicle manufacturers would list 3 to 4 different pressure recommendations based on vehicle usage.
#1.) around town driving
#2.) winter driving
#3.) high speed driving
#4.) fully loaded driving

Onto the Post-ERO times and the nine scariest words to hear

Our government decided that in our overly, litigation happy, society they needed to protects us from our own lack of personal responsibility. I mean can you really blame them. Nobody should be forced to actually read and understand their owners manual let alone follow some basic common sense rules. Our government enacted legistation called The Transportation Recall Enhancement, Accountability, and Documentation (TREAD) Act regarding recalls, tires and the related air pressure. Here are some of the highlights as it regards to this question:

#1.) tire pressure monitoring devices must be in all new vehicles sold in the US by 2008
#2.) there shall not be numerous pressures recommended by vehicle manufacturers
#3.) there must be tested to a 30% overloaded condition (previously it was 18% I think)
#4.) all tire DOT codes should be documented and recorded by tire manufacturers when the tire is sold to a consumer.

and in "congress speak" there is a whole lot of other BS in there as well.

as, Mike mentioned Mercedes has installed a placard recommending tire pressure but, per my #2 point, they can only recommend 1 pressure and they use their maximum high speed / high load recommendation

Now let's think about this
Driving at high speeds certainly helps make a trip go faster; just ask any driver who has gone "flat out" on the German Autobahn. However with the exception of events like the Silver State Classic's Open Road Rally or a driver's school on a racetrack, it's difficult to find a place that allows unlimited speeds! Remember, the tires on the vehicle should be properly sized, inflated and inspected if you plan to drive fast because the tires will be subjected to tremendous stresses.

Because of the weight they bear, pneumatic tires' sidewalls bulge and their treads flatten as they roll into contact with the road. This results in dimensional difference between the tire's "unloaded" radius (i.e., between the center of the axle and the top of the tire) and its "loaded" radius (between the center of the axle and the road). The engineer's call the difference between the two radii "deflection." Increasing vehicle speed will cause the tires to deflect quicker and increasing vehicle load will cause the tires to deflect farther (if tire pressure isn't increased).

So, unless all conditions and factors are not known there is no "correct" answer to your question

32 front and 30 rear should work fine on your car for normal driving conditions
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Last edited by Luke@tirerack; 02-02-2007 at 05:07 PM.
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Old 02-02-2007, 04:29 PM
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Wow Luke,....that was a mouth full.
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Old 02-02-2007, 04:31 PM
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Luke, I think you meant to respond to someone else. Most of your email seems pertinent until you state that my car is a 2006 and that I have uneven tread wear. My car is a 1988 420 SEL. My alignment is perfect. My tires are brand new from Tire Rack. That said, I am not an aggressive driver so 95% of the time my hwy speed is 75 MPH or less. I drive the speed limit in the city. Your follow-up feedback would be appreciated. Thanks.
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Old 02-02-2007, 05:07 PM
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Quote:
Originally Posted by Hans47 View Post
Luke, I think you meant to respond to someone else. Most of your email seems pertinent until you state that my car is a 2006 and that I have uneven tread wear. My car is a 1988 420 SEL. My alignment is perfect. My tires are brand new from Tire Rack. That said, I am not an aggressive driver so 95% of the time my hwy speed is 75 MPH or less. I drive the speed limit in the city. Your follow-up feedback would be appreciated. Thanks.

my bad ... I meant to edit out those two items ... but the pressure recommendation stands
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  #6  
Old 02-02-2007, 06:53 PM
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Location: DFW, Texas
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Luke, thank you. The folks at Continental USA stated 30 psi front / 35
psi rear using the Tire Guide as a reference. Although since you are the MB specialist, I place greater trust in your advice.

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