Tire PSI

MB recommends 30 PSI in the front tires of my SL and 33 PSI in the rear. Why 3 PSI more in the rear? It seems odd.

very good question:

they give a bit of allowance for the fuel inside the tank and trunk load.

i wonder how much is the pressure on volkswagen beetle.

Twenty gallons of fuel and a tiny trunkload of stuff would weigh more than the V-12 engine and front end suspension? Not to mention the load on the front tires caused by forward weight shifting during braking.

Maybe it's because rich people buy these cars and can fill the trunk with gold? I bought it used. I have no gold.

true!

the fact is that the front tires takes more the load during braking, and if there is a bump somewhere, it is the front tire to hit it too.

i am wondering also why don't they just make it even.

also, it is good to keep in mind that less tire pressure will give, while not a dramatic increase, better traction. the lower pressure in the front tires also allows for more flex of the rubber, which is more pronounced in the front tires which do all the turning. keeping the back tires pressure higher will allow the rear drive wheels to turn easier with less resistance. the pressure difference between front and rear is actually a very common recomendation for most cars.

I'm picking nits as far as argument, but 30 PSI is normal for these tires. It's the rear tires that have additional pressure, not the front that have less (than normal). I've never seen such a recommendation for a street car with the same size tires on front and back. I'm not saying it's never been recommended (after all, it's the recommendation for mine), but I am saying it's not common, so I disagree with your main point.

The rear tires, with additional pressure over normal, will have less traction and wear unevenly. Why would you want to sacrifice tire longevity and traction on a street car for a little less rolling resistance on two of the four tires? It doesn't make sense to me. On a race car to meet certain track conditions sure. On a street car I don't think it makes sense. Maybe there is sense to the recommendation, but we haven't uncovered it yet.

Well, for what it is worth, my previous cars (Hyundai, Peugeot, Nissan, Toyota) all recommended a higher pressure for the rear tires (10% higher). Same tire sizes. I guess I have never asked why.

Mainly for a more comfortable ride. Although I personally set them all at the same PSI unless the customer requests exacting specs. or if the vehicle has sport tires e.g. AMG package.

How does over-inflating the rear tires produce a more comfortable ride?

What do you mean by "sport" tires in regards to tire pressure. More importantly, why would that make a difference for street use?

AMG tire packages are most commonly different sizes on front and rear so I'm not sure why you use that as an example for tires of the same size.

I did not mention over inflating the rear tires. It has been explained to me that a few PSI less in the front makes for a more comfortable ride.
"Sport" tires are much softer than "General use" e.g. "all season" tires and most people who have these vehicles tend to keep the recommended pressures in their tires.
As for AMG, They design the wheels for their "package", not tires, and come to think of it there are alot of AMG vehicles that have matching tires all around.

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I did not mention over inflating the rear tires.

But that's what this thread is about. Sorry for confusing you. 30 PSI is normal for my tires. The recommended PSI on my car is 30 PSI in the front and 33 PSI in the rear. That means the rear tires are 3 PSI over-inflated. Are you with me now?

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It has been explained to me that a few PSI less in the front makes for a more comfortable ride.

Under-inflate the front tires by a few PSI? For street use? That will cause uneven wear and decrease fuel economy, as I explained.

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"Sport" tires are much softer than "General use" e.g. "all season" tires...

There are other important differences, but that's another debate. I just wanted to understand on what you were basing your point.

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... and most people who have these vehicles tend to keep the recommended pressures in their tires.

Which vehicles are you talking about?

And who's questioning whether anyone follows the recommended PSI or not, anyway? Where did that come from? How do you know that most people tend to do anything? Is that a gut feeling you have? Have you taken a poll? Have you read someone's research? What?

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As for AMG, They design the wheels for their "package", not tires,...

By "tire package" I mean the wheels as well as the tires used on whatever their "package" is and however they put it together, silly doof. I doubt if you have a clue how they design anything, but you're not above making a good-sounding guess. Which came first, the chicken or the egg?

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... and come to think of it there are alot of AMG vehicles that have matching tires all around.

So a lot are matching but most are not. What's your point here? Regardless, the PSI is most commonly the same on front and back even when the wheel and tire package for a vehicle package, however designed and whatever separate pieces come first on whatever package you want to define, includes different sizes, except for farm tractors, construction equipment, and such. I apologize for sending you off on a tangent.

Sorry if I sounded like I was off on a "tangent". I intended no such attitude.
We are on the same page, so to say, about this tire pressure thing.
I was just relaying what I was told when I asked the same question when I started working for MBNA.
In reality the driving conditions and intended use are what indicate proper PSI on any vehicle. In my short 20+ years of automotive experience, I find that most people do not check their tires often enough.
As for the 'sport tire' thing. I'm constantly reminded of the lack of knowledge as people here in MI purchase a set of low profile rims and proceed to install a tire with a treadwear rating around 200 and then complain that it doesn't handle worth a damn in the snow! And a large percentage of people who store their vehicle tend to ignore the tires-hence flatspots.
I don't make it here as often as I like, but do enjoy this section of the forum. I am willing to discuss any Mercedes related subjects if anyone wants to Email me. I'm slow on the computer (especially if my 4 year old is around) but I try to respond to all requests.
Sincerely, Larry

I haven't questioned the need for the lower psi for the front tires until I read this thread. For my 89 300E, I have been keeping the pressures at the recommended 26/30 psi front to back. At first, I was uncomfortable with the low pressure in the front tires. I haven't had a ride with that low recommended tire pressure since the 60's. After driving the car for 6 months, I find that my front tires are wearing evenly but the back tires are wearing on the outside edges as if the tires were under-inflated. I promise I do keep close tabs on the tire pressure and I don't drive with a elephant herd in the back. Anyone else notice this type of wear.

Cheers, JDub