ALFAitalia

My2002 W209 CLK200 Komp runs 18 inch AMG 4 wheels. The rear tyres are 255-35/18 and the fronts are 225-40/18. Just wondering what pressures those of you running the same size tyres are running. The sticker in the fuel flap only gives pressures for the stock 16 inch wheels with balloon tyres!!! LOL. Obviously looking for the right compromise for wear, handling and comfort.


Also, for the future as my tyres are only a couple of thousand miles old, what size tyres fit with stock height and type suspension. There seems to be plenty of space in the rear but the tread surface only misses the front spring perch by less than an inch. I would be willing to sacrifice a little handling sharpness for a touch more comfort.....if the tyres fit. In NOT willing to change my wheels though....love the look of them!

Fatz

Without looking at the stickers on my car, the pressures are 38psi front / 39psi rear. This is a bit too "comfortable" for me. I generally run them 39/40 locally and 40/41 for trips over 100 miles. (Same size tires)

Yidney

My door sticker says 38F 42R, so that's what I do.

Fatz

Do you think the 42R may have to do with your car being a cab?

xsever

Definitely, because the rear if heavier than the coupe (weight of the top) so the center of gravity is more aft.

ALFAitalia

All those pressures are rather higher than expected. I checked the pressures for my model/tyre size on various tyre manufacturers websites and they are all coming up with 31-32 front and 34-34 rear. I guess all you guys would think that's wrong or is it just your personal tastes?

sailorben

So on my 07 with 17" 225 and 245 and sports suspension.

I run pressures at 31psi on both axles.


In my fuel filler cap, it states tyre size as the 18" rims which confused me, as on a vin decode. I have 17" fitted, from factory. Anyway...

So I'm running 31psi for normal running. If I take the pressures up to 34 which I have done using a crappy air compressor at a petrol station when I was caught without one; the ride was noticeably harsher.

But back down to 31psi now and that provides just a little more comfort. I can take a pic of the filler cap if you wish. I believe it has the 18" 225/255 stamped above there.

ALFAitalia

Yeah....my sticker says 31 front and rear .....but makes no mention of which tyre size that refers to. But I have my build sheet that states the car was built with 16 in rims so I can only assume the 31 psi on the sticker refers to 16 inch higher profile tyres.

sailorben

Here you go:

sailorben

So actually I was misunderstood. Actually looking at it *properly* it says +3psi for your size 18-225/255.

DanielFD

I had 19" on my CLK. i ran 36psi front and 38psi in the back.

insame1

Don't even look at the sticker if you Change your rim diameter and width. The best thing you can do is get a tire thermometer but a instant read will work and some chalk. Put what you think is close to the correct amount of pressure in the tires and go for a ride. What you are looking for is fairly even heating across the contact patch. Add or remove pressure until it is even. more if the center is cooler and less if opposite. Then take the chalk and rub it on the side of the tire from the tread up the sidewall to the line or text. then go for another drive and inspect the side wall. See how high up the chalk has been removed and adjust your pressure till it is correct. Try to find a balance between the two test. Any other suggestions are speculation.

ALFAitalia

Good advice....but the sticker above clearly says the extra pressure required in my size wheels and tyres as its factory option size (strange how my sticker does not) so that will be my starting point and I'm sure, with Mercedes having tested it with that size, it wont be far away. So that's 32 front and 33 rear. Thanks all.

Fatz

The profile of the tires will also factor into the correct tire pressure setting. I guess that's another reason the OEMs put those stickers on the cars. A long time ago I did some research and calculations to come up with the 39/40 local and 40-41/41-42 extended highway values I use.

Go with the OEM recommendations where you can..... Changing the wheels and tires puts you into coming up with your own, or what someone else has done. You're doing the right thing... IMO

insame1

This assumes that you are running the same tire on the same width rim with the same offset. Granted it is not going to be a night or day difference +/- a few psi.

A lot of people do think about things that change when you go away from the factory setup. For instance if you push your wheels out with offset, spacers or just a wider rim you are effectively changing the spring rate.

ALFAitalia

Why would you put the tyre pressures up on the highway? I thought that was a bit old skool thinking. Tyres get hotter at sustained high speeds and therefore the pressure increases naturally. Most folks no longer change their pressures for fast driving anyway......but if they do they really want to slightly lowering the pressure IMO.


EDIT. Having looked at various tyre manufacturers websites they mostly DO say to increase pressure with speed to minimise side wall deflection and therefore increase stability.....none make any mention of the natural psi increase with speed/heat which is quite significant. Measure your pressures on a cold day then a hot day (even without driving it) and you might be surprised at how much higher the hot day pressures are. Interesting.

ALFAitalia

True although it depends on the individual cars suspension design....my solid axle Jeep will not alter the effective spring rate no matter how far I push the wheel out (at least on a bump hit by both wheels on the axle at the same time.....but most modern cars will. The spring rate of course remains unchanged but pushing the wheels out gives more leverage over the spring therefore making it feel softer.......by an amount that would be virtually imperceptible though! And much against popular belief pushing the wheels out will NOT affect body roll as the roll moves around the springs....unaffected buy wheel position. You would need to move the springs in or out from the centre line of the vehicle for that to happen....

Fatz

The tires get hot primarily because of the number of times the tires are flexed per unit of time. They will flex more with less air pressure. The hotter they are, beyond a point, the less traction you will have...... responsiveness and control will decrease because the hotter tire will flex more easily laterally..... making evasive maneuvers more difficult (there are no threads on the sidewalls)..... and you are more likely to get wheel damage if you hit debris on the highway with less air...... and the lower pressure tire creates drag reducing mpg..... and a tire with less flex is less likely to fail at high speeds due to less stress between the tire layers... And yep, I am old school..... but it's ok, I'm an old guy..

insame1

What are you even talking about? If my car was a magic carpet I wouldn't eve need tires or springs.

Fatz

The increase in tire pressure under operation (normal) is taken into account by the manufacturer when setting the "cold" air pressure value. The tires heat up fairly quickly. I believe they target a temperature range whereby the tire will perform to specs with a recommended "cold" tire pressure. Hence, one should check and adjust tire pressure at least twice a year where the seasonal average temperature fluctuates by more than say 30 degrees (F) (the 30 degrees is a guess)..... This is also why there is a "max" pressure listed for most tires. The tire probably won't rupture while not in use with the "max" exceeded by xx%, but it is likely under operation with tires worn 50% or more.

Glyn M Ruck

For majority 1 up no load I run 2.3 bar front & 2.1 bar rear. This gives absolutely even wear of tyres. Any harder at the rear & I suffer centre of tread wear. Stock 17" 245/225 combo.