I recently bought new tires for my 2002 E320. The tire dealer set the pressure at 40psi as the max rated psi for the tire is 44. He said they will run cooler and therefore last longer. However, the car rode like a truck and the handling seemed almost unstable at interstate highway speeds. So, I lowered it to MB's recommended full load pressure of 28 front and 34 rear and the ride and handeling are now as they should be.
My question, who's really right? Seems that tire dealers and auto manufactures have never agreed on the correct psi.
The correct tire pressure is the one from Mercedes Benz. The tire manufactures give you the maximum tire pressure that is allowed for the tire. Your vehicle has a pre-determined weight and requires a certain amount of tire pressure to maintain ride quality. If your vehicle recommends that your tire pressure be at 30PSI and the tire maximum pressure is 45PSI and you fill the tire to the maximum, you will have uneven wear (wear pattern will be thin treads in the middle and normal tread on the outside edges). The tire company will make a tire and not know what car it is going to be installed on, however the auto company knows the perfomance demands on their car, and will install a tire rated to meet their standards. That is why tires companies have standarized letter ratings.
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how about the size of your wheels and tyres? does that affect the tyre pressure? eg: say yuor merc comes with stock 15' wheels and you put on 18' with low profile tyres...should the tyre pressure be the same or different?
First of all, never run your tires at or even near the max rating on the sidewall! And, depending on the type and size of the tire, you may not want to run it at exactly the MB recommendation inside your gas tank door. For example, mine recommends 29F and 32R for the 16" OEM all-season tires, but when I bought new tires from the Tire Rack, Luke recommended I run the 18" Goodyear Eagle F1's at 36F and 34R, and my 16" Michelin X-Ice at 34F and 32R. The car rides like a dream and handles even better!
The tires do make a difference, and you need to be able to depend on some tire professional for good information.
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I like to run 35psi front and 38 psi rear. I'll live with the decrease in ride comfort, and my wallet will appreciate the lower fuel consumption and longer tire life.
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Respectfully,
/s/
Max
Charleston SC
'87 300TD, 294k miles
'73 Balboa 20
MBCA member #D008916
Join the Mercedes-Benz Club of American today, and get discounted parts at your local MB dealership: MBCA
As you can see from the recommendations from Tire Rack, probably slightly higher pressure up front is preferable because the car will be nose heavy. The rule of thumb is that you increase pressure as load increases. Pressures need to be higher on very thin side walls, ie: big wheels and low profile tires. If you look at the sidewall ratings, the max pressure listed is also at the max weight rating for the tire. Since your car never approaches the weight limit of the tires, pressure should go down accordingly. Over inflating will barely increase mileage, but it's still at the expense of tire life, it doesn't help. All you'll get is tires worn in the middle and less so on the sides. In the end, the correct pressure is that which gives the most even (and longest) tire wear. On a 285/20, I'd probably start around 36 psi and go up from there until it tends to wear in the middle and/or ride quality is rough. Anything lower than that and you could be risking wheel damage if your roads aren't smooth. Everyone should take the time to experiment with pressures a little, you might be surprised at what actually works best for you and your car. I'll bet it's not what's on the sidewall.
I went from OEM 16" 225's to 245/45WR-18 Goodyear Eagle F1 GS-D3 XL front and 275/40YR-18 Goodyear Eagle F1 GS-D3 back, and Luke recommend 36 psi front and 32-34 psi back. You probably need to go a good bit higher w/ 20"s.
The correct tire pressure is the one from Mercedes Benz. The tire manufactures give you the maximum tire pressure that is allowed for the tire. Your vehicle has a pre-determined weight and requires a certain amount of tire pressure to maintain ride quality. If your vehicle recommends that your tire pressure be at 30PSI and the tire maximum pressure is 45PSI and you fill the tire to the maximum, you will have uneven wear (wear pattern will be thin treads in the middle and normal tread on the outside edges). The tire company will make a tire and not know what car it is going to be installed on, however the auto company knows the perfomance demands on their car, and will install a tire rated to meet their standards. That is why tires companies have standarized letter ratings.
I always use the tire pressure recommended by Mercedes as specified on the fuel filler flap.
Goodyear's may have a max pressure of 40 psi and Bridgestone may be 38 psi , This is the Max pressure for the tire.
The max pressure may be a hard ride for your car, as this is higher than the recommended pressure for the car.
As you can see from the recommendations from Tire Rack, probably slightly higher pressure up front is preferable because the car will be nose heavy. The rule of thumb is that you increase pressure as load increases. Pressures need to be higher on very thin side walls, ie: big wheels and low profile tires. If you look at the sidewall ratings, the max pressure listed is also at the max weight rating for the tire. Since your car never approaches the weight limit of the tires, pressure should go down accordingly. Over inflating will barely increase mileage, but it's still at the expense of tire life, it doesn't help. All you'll get is tires worn in the middle and less so on the sides. In the end, the correct pressure is that which gives the most even (and longest) tire wear. On a 285/20, I'd probably start around 36 psi and go up from there until it tends to wear in the middle and/or ride quality is rough. Anything lower than that and you could be risking wheel damage if your roads aren't smooth. Everyone should take the time to experiment with pressures a little, you might be surprised at what actually works best for you and your car. I'll bet it's not what's on the sidewall.
thanks fast. appreciate your advice...
i will play around to see what works good for me.
but i didn't pay attention to this thin sidewall tire business until yesterday.
i used to drive it 29 and 32.
the other day i went to put 2 new tires and guys in shop educated me a little bit about differences between "regular" and high performance thin wall tires.
Correct Tire Pressure 
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