Do I have the wrong tires on?

[Fon]

W221, 2007 (pre face lift.)
Cd 320

Rear, 285/40/R19
Front 255/40/R19

I ask becuase I cant find the correct tire pressure for the 285 anywhere online, and im also experiencing quite a firm ride (can feel lots of bumps)

Many thanks,

Fon

 

[grober]

There's a mismatch in tyre+wheel rolling diameter there- think your rears should be
285 35 R19

SEE
http://www.carbibles.com/tyre_bible_pg4.html

 

[markjay]

As above,

If the tyres are all /40/R19, but have difference widths, then the two rear tyres would be taller (i.e. have larger diameter) than the front ones.

You want to have wider tyres at the rear, but keep the same tyre height (diameter), and this is achieved by having lower profiles on the (rear) wider tyres.

 

[markjay]

Having larger tyres on the rear will affect ride quality, will potentially cause erroneous speedometer reading (depending on whether the speed reading is picked-up at the front hub, rear hub, or transmission...), will result in higher gearing which will improve economy and top speed but at the expense of acceleration, and might confuse the ABS/ESP system.

 

[amiller81]

Tyre sizes and pressures should be on the inside of your fuel filler flap

 

[SLKGirl]

As above,

If the tyres are all /40/R19, but have difference widths, then the two rear tyres would be taller (i.e. have larger diameter) than the front ones.

You want to have wider tyres at the rear, but keep the same tyre height (diameter), and this is achieved by having lower profiles on the (rear) wider tyres.

I'd always wondered why the rear tyres had a lower profile (i.e 35 not 40) than the fronts.

 

[markjay]

I'd always wondered why the rear tyres had a lower profile (i.e 35 not 40) than the fronts.

Because they are wider....

But this is only applicable to 'staggered wheels' layout, i.e. where the rear wheels are wider than the fonts.

Not applicable to my (non-Sport / non-Avantgarde / non-AMG) Classic SE car that has 4 identical tyres

 

[Fon]

There's a mismatch in tyre+wheel rolling diameter there- think your rears should be
285 35 R19

SEE
Car Bibles : The Wheel and Tyre Bible

Thank you.

It will become clear I know nothing.

Would my wheels be the same size?

IE if I change both rear tires, could I keep the old tires from the rear, to put on the front when they go bald?

Fon

 

[grober]

Sorry NO your front wheels will have narrower rims than the rear 8 1/2 J FRONT 9 1/2 J REAR if they are standard MERC
Have a read of the link I posted it will explain everything.

 

[markjay]

As above

Said that, while in essence the wider/lower tyres will require wider/taller wheels, if the wrong tyres were fitted on the original MB wheels, i.e. the car still has the original factory-fitted alloys, then you are in luck and you can keep the wheels, just replace the tyres with the correct ones.

 

[Fon]

As above

Said that, while in essence the wider/lower tyres will require wider/taller wheels, if the wrong tyres were fitted on the original MB wheels, i.e. the car still has the original factory-fitted alloys, then you are in luck and you can keep the wheels, just replace the tyres with the correct ones.

Thanks,

How could I tell if I have the original wheels and wrong tires?

Fon

 

[markjay]

If you remove the wheel, you will see the size stamped on the inside, e.g. 7.5J ET34.

You can then compare this figure to that of the original factory fitment.

But you will need to check your car manual to know what was the size of the original wheels supplied with the car.

It will also tell you what size tyres fit the original-size wheels.

 

[AkisGr]

The fronts dimension is right.The rears are a tiny bit taller profile than OEM.
In fact the difference is only 4mm,which aint enough to cause problems.
The rear profile hight with the OEM tyres (275/40/19) is 110mm and the tyre that you have now (285/40) is 114mm.

Your harsh ride is not because of the tyre dimensions.
What is the cars weight and what is your current pressure on the tyres?

It could be tyre pressure as you say. But it can also be that you need to change some bushes,balls and joints on your suspension.That makes a big difference to ride quality.

About a suggestion above...You don't need to make the rears 285/35. This is the Brabus setting and its only 100mm tall. You now have 114 mm profile on the rear ,which is better for comfort.
If you want to be 100% correct on OEM settings,i would suggest you go to 275/40 (110mm) instead of 285/35(100mm).This would keep almost the same hight of the tyre with reduced footprint,which would be better for ride quality and noise.

 

[AkisGr]

I'd always wondered why the rear tyres had a lower profile (i.e 35 not 40) than the fronts.

It's not about rear and fronts being equaly tall. In fact you'll have the exact same hight only if you have the exact same tyres for front and rear.
And that doesnt happen in most cases.

Anyway, the 35 or 40 that you see as the profile indicator ,is not milimeters. It's the percentage of the width of the wheel.
In a 285/40/19 tyre:
the 285 is 285 mm width,
the 40 is 40% of 285mm (meaning 114mm tyre profile) and
the 19 is the diameter (as u know) of the tyre ,in inches.

 

[renault12ts]

The fronts dimension is right.The rears are a tiny bit taller profile than OEM.
In fact the difference is only 4mm,which aint enough to cause problems.
The rear profile hight with the OEM tyres (275/40/19) is 110mm and the tyre that you have now (285/40) is 114mm.

Your harsh ride is not because of the tyre dimensions.
What is the cars weight and what is your current pressure on the tyres?

It could be tyre pressure as you say. But it can also be that you need to change some bushes,balls and joints on your suspension.That makes a big difference to ride quality.

About a suggestion above...You don't need to make the rears 285/35. This is the Brabus setting and its only 100mm tall. You now have 114 mm profile on the rear ,which is better for comfort.
If you want to be 100% correct on OEM settings,i would suggest you go to 275/40 (110mm) instead of 285/35(100mm).This would keep almost the same hight of the tyre with reduced footprint,which would be better for ride quality and noise.

It is all about the tyres being the same height. The rears in the OP's case have the wrong profile and therefore they are too tall when compared to the fronts. If you have 255/40 on the front then you should have 285/35 on the back...because then they are almost exactly the same diameter (being only 4mm difference on the radius).

I think you'll find the rears are 24mm ( an inch in old money) taller than the fronts. This is far too big a difference. I would be surprised if the cruise control worked...that's hardly a tiny difference as you state.

The difference between 255/40 and 275/40 is 8mm in the radius, not 4mm as you state.

So, the suggestion for 285/35 was indeed correct.

 

[AkisGr]

It is all about the tyres being the same height. The rears in the OP's case have the wrong profile and therefore they are too tall when compared to the fronts. If you have 255/40 on the front then you should have 285/35 on the back...because then they are almost exactly the same diameter (being only 4mm difference on the radius).

I think you'll find the rears are 24mm ( an inch in old money) taller than the fronts. This is far too big a difference. I would be surprised if the cruise control worked...that's hardly a tiny difference as you state.

The difference between 255/40 and 275/40 is 8mm in the radius, not 4mm as you state.

So, the suggestion for 285/35 was indeed correct.

I think you should read my posts agains carefully. There's no reason to confuse the OP.

First of all,i said its only 4mm difference between the OEM (275/40) and his tyre (285/40)

And second, If it was all about same hight,Mercedes wouldn't have charts with 8mm profile difference between front and rear. (255/40 front with 275/40 rear is the Mercedes OEM dimensions).So the OEM dimensions have 8mm difference between front and rear already.With the tyre he has now,that difference is 12mm.It's not the correct one,but it's not what causes him the issues he's complaining.

What i simply said is go for the Mercedes OEM settings and not the Brabus settings (285/35 that you propose),because the OEM's smaller footprint and larger tyre wall will have much better ride quality.

And in the end,even if we supposed falsly that OEM tyre hights are even,how would that solve OP's issues? Hight difference between front and rear wheels has nothing to do with OP's problems.
Firm and harsh ride has nothing to do with that.

 

[markjay]

I do not know whether having the wrong tyre size is the cause of the OP's issue.

But I do know that wider tyres have lower profiles in order to keep the height (diameter) as similar as possible. It is as simple as that.

If you do not accept this statement, then we will have to agree to disagree.

 

[AkisGr]

I do not know whether having the wrong tyre size is the cause of the OP's issue.

But I do know that wider tyres have lower profiles in order to keep the height (diameter) as similar as possible. It is as simple as that.

If you do not accept this statement, then we will have to agree to disagree.

It's not a contest of statements. It's about helping the OP with his specific problem. Not with what we have in mind saying.
And yes,going from 114mm profile he now has,to 100mm as the other member suggests, in the rear ,will not only make no difference to OP's problem,but will infact make it worse. It's simple to understand.

Mercedes OEM is 255/40 front and 275/40 rear. Which is 102mm front profile and 110 mm rear profile. From here on we can all just do the math.

Anyway,the differences we talking about would MIGHT matter for handling and functioning(NOT FOR RIDE COMFORT) on a 4 wheel drive,not a 2 wheel drive car.

PS: "But I do know that wider tyres have lower profiles in order to keep the height (diameter) as similar as possible.".....We always need to understand the reasons of something and not just hear and adopt a statement.We first need to understand the reason for the wheels being the exact same or not diameter.
In the specific case,you don't need to understand something.
All you need is follow Mercedes's guides and use your logic with OP's specific parameters.

 

[markjay]

... It's about helping the OP with his specific problem..... All you need is follow Mercedes's guides and use your logic with OP's specific parameters.

Agreed.

The OP asked if he has the wrong tyres for his car. The answer is yes, he does not have the tyres that MB recommend for his car.

The OP asked if this could be the reason for his problem. My answer was that I do not know.

I think I tried to be as helpful as I can.

 

[grober]

I had noted that the standard rear wheel tyre size was 275/40/19 but felt that this might confuse the OP even more by mentioning it. Running a 285/35/19 will alter the gearing slightly from standard but what may be of more importance is the actual rim width in relation to the tyre width-- and of course we are assuming the OP is running 9.5J on the rear. The relationship betwen the tyre width and wheel width is an important one in so far as if effects the effective tyre wall height. E.G. If you fit too narrow a tyre part of the effective tyre wall is used to "stretch" that tyre to fit on the rim- this may have a marked effect on tyre sidewall compliance. Too wide a tyre and the tyre acts more like a balloon with perhaps a similar effect in the opposite direction.
So first the OP has to determine the actual rim width of his 19 inch rear wheels- if they are 9.5J [std 19inch merc] then probably best to go for the Merc recommended 275/40/19----- and then run them at the lowest recommended pressure--- Merc have recently taken to using very high "recommended" pressures= higher than the manufacturers recommendation for load bearing and speed ratings simply to achieve better official fuel consumption /emission figures