Tyre Pressures

[Beardz]

I have just gone from 16" wheels - 225/55/R16 to 18" wheels 245/40/R18 front & 265/40/R18 at the rear. Can anyone tell me what tyre pressures I should be using?

Do I use the pressures on the inside of the fuel flap (which I assume are for the 16"s only)?

I've hunted about and cannot find anything (tried MB website & the owners manual) even for the sport which i think uses the same size wheels/tyres.

It's just that apart from making sure i'm not wearing the tyres out too fast I am going to be using the car fully loaded (4x adults, 2x dogs, associated luggage and roof box) and I definitely want the correct pressures for then.

Now I suppose I could pick up the phone and ring MB but seeing as I don't have a good relationship with them I thought I'd ask your good selves first.

 

[renault12ts]

As for the 16s.

 

[Beardz]

As for the 16s.

Ok, really? Is that a general thing or just in my case (for future ref)?

Being a mechanical ignoramus I just assumed that there would be a difference.

 

[MB-BTurbo]

I would be very surprised if it was the same pressure for the 16s. They are more often than not a higher pressure. There is far less give than in 16s so if you run over a pothole in 18s with 16s pressure you could ruin your alloys.

 

[renault12ts]

And so it goes as ever...opinion over fact. I was stating fact.

 

[MB-BTurbo]

You probably are, it might just be coincidence that each car I have had has an increase pressure coinciding with an increase in tyre diameter. Also on my B class there are pressure charts for 17" and 18" tyres. The 18s are of a higher pressure!

 

[HB]

29 at the front and 32 at the back.
That's what it says on my fuel flap and mine is on original 18 amgs

 

[renault12ts]

You probably are, it might just be coincidence that each car I have had has an increase pressure coinciding with an increase in tyre diameter. Also on my B class there are pressure charts for 17" and 18" tyres. The 18s are of a higher pressure!

The diameter of the tyre will not increase as the diameter of the wheel increases (assuming we are talking about the car remaining the same).

 

[MB-BTurbo]

No, which is where my assumptions made a fool out of me. Either way why would my fuel pressure filler cap state a lower pressure for smaller tyres?

 

[renault12ts]

 

[whitenemesis]

Tyre pressure is a function of axle load, not wheel/tyre sizes.

Sometimes it is suggested to run lower profile tyres at the higher end of the stipulated range as extra protection against 'snake-bite' damage.

 

[MB-BTurbo]

What is 'snake bite' damage?

 

[whitenemesis]

Terminology taken from mountain biking

Basically where the tyre gets pinched between the rim and an edge object, say, pothole in the road or the edge of a kerb

 

[Beardz]

So we are agreed that I should run the same as the 16"s including the higher pressures when fully loaded.

 

[Beardz]

Will the speedo be affected by the difference in wheel or is the car smart enough to work it out itself?

 

[renault12ts]

Will the speedo be affected by the difference in wheel or is the car smart enough to work it out itself?

The wheel may be a greater diameter, but the tyre will be the same, so the speedo will not be affected.

 

[whitenemesis]

If you have fitted the correct tyres then the rolling diameter will be unchanged and the speedo reads as before.

Looking at the tyres now fitted the fronts are ok but the rears probably should have been 35 profile i.e. 265/35/18

 

[nigel cross]

It will change and your speedo will be wrong. Have a look at link below.
TRC Calculator

 

[renault12ts]

It will change and your speedo will be wrong. Have a look at link below.
TRC Calculator

No, it will not. If you put on a greater diameter wheel, you put on a tyre with a lower profile to compensate and thus have the same diameter.

Is this so difficult?

Use this calculator to compare sizes.

http://www.rimsntires.com/specs.jsp

 

[C43AMG]

On my W221 i have 20" and on the petrol cap it says increase by + 3 psi.