Is it within Mercedes Specification to fit 255/40/18 - 99Y MO

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Sammerc350

Member
Joined
Nov 11, 2019
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45
Location
Wiltshire
Car
Mercedes S350 Bluetec
Could someone kindly confirm if it’s within Mercedes Specification to fit

255/40/18 99Y MO - To my S350 Bluetec - 2012

The standard Tyres I have on the Front are 255/45/18 99Y MO - Which are Perfect!

“I’m only looking at changing the 2 Rear Tyres”

If anyone could let me know if the above is ok, and what difference this would make,
it would appreciated 👍
 
Last edited:
Could someone kindly confirm if it’s within Mercedes Specification to fit 255/40/18 99Y MO - To my S350 Bluetec - 2012

The standard Tyres I have on at the moment are 255/45/18 99Y MO -

If anyone could let me know if the above is ok, and what difference this would make, it would appreciated 👍
@Sammerc350 I don't know if it's ok, but there is an online tyre calculator that shows the difference made to the speedo measurement when you put a different tyre on compared to factory spec
 
Definitely not. A 255/40 is a smaller diameter tyre than a 255/45 - by about 25mm if I have worked that out properly in my head.
It is not a good idea to move away from the recommended size and an even worse idea to mismatch front to rear.
An insurance assessor would have something to say about the incorrect tyres if the car was involved in an accident.
Why would you even think of doing it?
 
5mm difference in the sidewall
Definite no no ,not on the rear
 
Thanks for the replies, reason I ask the question I was offered 2 brand new with labels Continental MO Tyres, excellent price, so just wanted to check and see if it would be safe to fit them.

I see from the comments that it won’t work, so I will pass on them.
 
Im sure you could put the 40's on the front and the 45's on the rear
 
If you change the sidewall profile then you should also change the width of the tyre so that the overall diameter remains the same - or very close. Staggered set ups use this approach - so a 255/35/18 is the same diameter as a 225/40/18 - and the lower profile goes on the rear not the front. Then bear in mind that the lower profile (and wider) tyre may need a wider rim - so it really is not as simple of deciding to swap a 40 to 45 sidewall, especially on one axle - you would then have a car running different diameter tyres front to back.
 
If you change the sidewall profile then you should also change the width of the tyre so that the overall diameter remains the same - or very close. Staggered set ups use this approach - so a 255/35/18 is the same diameter as a 225/40/18 - and the lower profile goes on the rear not the front. Then bear in mind that the lower profile (and wider) tyre may need a wider rim - so it really is not as simple of deciding to swap a 40 to 45 sidewall, especially on one axle - you would then have a car running different diameter tyres front to back.
You can run different diameter tyre's front and back?
 
If you change the sidewall profile then you should also change the width of the tyre so that the overall diameter remains the same - or very close. Staggered set ups use this approach - so a 255/35/18 is the same diameter as a 225/40/18 - and the lower profile goes on the rear not the front. Then bear in mind that the lower profile (and wider) tyre may need a wider rim - so it really is not as simple of deciding to swap a 40 to 45 sidewall, especially on one axle - you would then have a car running different diameter tyres front to back.

Many Thanks Smiley for the explanation, it’s much appreciated 🎩
 
You can run different diameter tyre's front and back?
Well I suppose you "could" but it would be highly unusual for a car; bit like a tractor.
Maybe you assume that different size rims or different sidewall aspect ratios mean that a tyre is a different diameter?
As an example the following tyre sizes all have the same diameter (to within small tolerances within the wear of a tyre)
195/65/15, 205/60/15, 205/55/16, 215/50/17.
None of those numbers tells you the diameter of the tyre - you work it out from the diameter of rim (15 or 16 or 17 inches) plus the size of the sidewall eg (195 x 65/100)mm; times 2 because there are two sides of the sidewall in the diameter then add that to the 15/16 or whatever rim size (after converting the inches to mm).
Simple :rolleyes:
 
I meant different size wheel's, as for working it all out ill let somebody else do that🙂 what ever it was made with is what its having,apart from brand😃
 

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