Oversized tire pressure

tires

#1

I recently purchased a used 2004 Mercedes S430. The previous owner upsized the wheel rims and tires from 18" to 20". Should I add more PSI to these tires or stick to what the MB sticker states?


#2

Think of it this way: the thing that supports each wheel and keeps your car off the ground is a “tire patch”, which is usually about 60 square inches. If your tire is inflated to 30 psi, it’s providing 900 pounds of support (30 X 60). If your upsized tires have a larger tire patch, maybe 70 square inches, and they have the same weight to support, then, I suggest you reduce the pressure by a proportionate amount. Basically, you’re looking at a lower pressure not higher. You probably want to talk to a knowledgeable tire retailer for specifics.


#3

Sorry! 30 X 60 1800 lbs, but you get my drift.


#4

Which rim size is stock? I would guess the 18" is, in which case you follow the door sticker recommendation.

Anything else will be different and your vehicle may not handle as well.


#5

I bet your car rides like a tank with those 20" wheels…


#6

Actually, the MB drives like a dream no matter what PSI I put in the tires (I have never put more than 3 psi more than the original MB states for 18" tires). In fact, even though I loved my BMW 740iL, which I had for almost 6 years–it was a 1998 model–driving the MB is like driving on air. It also, like my BMW, has perfect steering control and I can weave in and out of traffic (I live in LA) as though it were a smaller sports car. Even though I loved my 740iL, I think I love my new MB S430 more. Even though they are similar curb weight, my MB breezes, while my BMW “felt” the pavement. I know this doesn’t add anything about my question as to tire pressure, I simply want to let you know that the MB “feels” like a lighter car than the old 740iL, even though the driving experience is as excellent.


#7
For the correct pressure, I would try TireRack or the tyre manufacturer for recommended pressure for that tyre and car.  The MB sticker no longer applies, but it may well happen to be correct. 

I would tend to error on the high side, especially if you live in a pot hole part of the world.  Those fancy rims and skinny tyres don't offer much protection from road damage and you can easily loose not only the tyre, but the rim also.

#8

The correct method to determine the pressure is to match load carrying capacities. You need three bits of information - the original tire size as listed on the vehicle placard, the pressure specified by the vehicle placard, and the tire size you are currently using - AND the load tables for the 2 tire sizes involved (which are hard to find on the internet).

If you’ll post the tire sizes and the pressure on the placard, I have the load tables, and I’ll do the calculation for you.

BTW, 03impreza is wrong about the pressure / contact patch area thing.


#9

Hi CapriRacer,

Thanks alot for your offer to help. The recommended tire size is 225/55R17 97H, with 29psi front and 32psi rear. What is on now are Nitto NT555 275/30ZR20 97W’s. The rims are Lowenhart (I think 20"). Again, thanks.


#10

Probably not the answer you’re looking for, but why don’t you try to track down a set of the correct wheels next time you need to buy tires.


#11

Markaq55,

I answered your question on another web site.

But for those that are interested: 6 psi more than what the placard says: 35 front / 38 rear.


#12

Thanks again.