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.