Correct tyre pressures

[Adeinfrance]

Hi. Can anybody advise me of the correct tyre pressures front and back for 225/45/17 tyres on 8J x 17 alloys. My local garage has put in 2.5 bar all round but I just thought this was a bit too much although the car drives ok.

 

[markjay]

What car / what year?

 

[Adeinfrance]

It's a 2002 W208 230K CLK.

 

[Dieselman]

2.5 Bar sounds rather high.
The standard pressures will be 2.0 bar front and 2.2Bar rear. The pressure chart should be in the fuel filler flap.

 

[Adeinfrance]

They seem a bit high. The wheels and tyres are not the standard ones so wasn't sure if the pressure stays the same or not for different size tyre/wheel combinations.

 

[Dieselman]

Did the weight of the car alter?

 

[Paul/]

Is the standard tyre a 225? If not, should pressure be lowered in inversion proportion to the increase in width over standard?

 

[Adeinfrance]

The old wheels and tyres were the original 16" CLK alloys but don't know what size the tyres were as they are already gone. No change to the weight of the car.

 

[Paul/]

I was thinking- don't know if there are other considerations though:

Pressure = Mass / Surface Area.

Mass (weight of car) is unchanged.

So if new tyre is for example 5% wider than standard, contact area will be 5% greater- so would need 5% less pressure to support the same weight.

 

[Bobby Dazzler]

The manufacturer guidelines on tyre pressures (see fuel filler flap) account for a range of wheel and tyre sizes, so use those for now. They are only a guide even for standard wheels, and you need to adjust based upon wear or they way they feel when driving.

If the wheels are significantly larger than the largest factory size (18 inches in diameter I think on the CLK) then you can adjust further. Best keep a close eye on wear pattern if more than 19 inches.

 

[Dieselman]

I was thinking- don't know if there are other considerations though:

Pressure = Mass / Surface Area.

Mass (weight of car) is unchanged.

So if new tyre is for example 5% wider than standard, contact area will be 5% greater- so would need 5% less pressure to support the same weight.

The contact patch remains the same as the mass hasn't changed.

 

[Paul/]

The contact patch remains the same as the mass hasn't changed.

The contact patch has got 5% wider if the tyre is 5% wider.

...I think- but perhaps a wider tyre under the same load also has a reduced LENGTH (along axis of rotation) of contact patch as a result, though I'd guess that this wouldn't be significant compared with the 5% increase.

 

[Dieselman]

The contact patch has got 5% wider if the tyre is 5% wider.

...I think- but perhaps a wider tyre under the same load also has a reduced LENGTH (along axis of rotation) of contact patch as a result??

Which is why wider tyres aquaplane more readily.

 

[Paul/]

Certainly it's a commonly held view that contact area is simply proportionate to load & air pressure.

But here's an article (I can't vouch for its credentials) that concludes that wider tyres probably have larger contact area

Fact or Fiction? Tire contact patch and air pressure.

...anyway, this is just for those interested. I don't recommend my own original advice to the OP if it's add odds with consensus.

 

[ALNoteman]

Tyres, Tyre pressure and wheel sizes?

Hi All,

I'm reading your posts on tyre pressure etc. I have a similar problem, my 2003 C-270 diesel Elegance Estate had the wheels and tyres changed from 16-inch to 17-inch by the preveous owner and to start with these had a pressure all round of 41-psi. The ride was very hard so I reduced this to 35-psi and it became a little softer ride! My present tyres are thr 17-225-45 but I would like a bit more wall than this but not sure just how much larger a tyre I could fit on these rims on this particular car so if anyone has data similar to what I just read about an SLK where the poster knew the maximum wheel size that the car was designed for but for my C-270 mentioned above, I would love to hear from you.
Al.
[email protected]

 

[grober]

FOR A C203 270 diesel estate 2.2 bar [ 32 psi] front and 2.4 bar [ 35 psi ] rear would be a good starting off point for a lightly loaded 225/45 R17 Y/Z 94 XL tyre imho. I would add at least 3 psi front and 5 psi rear for prolonged high speed or a heavily loaded car. Without knowing exactly the load/speed rating of the tyres its difficult to say exactly.

 

[ALNoteman]

Tyres and pressures

Thanks John and others on the subject of correct tyre pressure.
I have done as advised and made sure that I have at least 35-psi in the front and 38-psi in the rear as I live right on J22 of the M1 so a lot of my driving has to be at max legal speed otherise I'm holding everyone else up.

I was also worried about driving on snow and ice as I've been driving AWD or 4X$'s for the past 10-years and heard that the C-270's are not so good on ice and snow? I've just been out to Halfords and bought a pair of Easy-Grip snow chains, to me these are new and made from composite materials instead of steel. They are made by Michelin in France and cost £99.99 a pair. Has anyone had any expeience with these or steel ones. I thought composite would be kinder on tyres and alloys?

Al.

 

[grober]

I reckon the Easy-Grips are fundamentally a good idea but being made of composite may wear quite rapidly on tarmac or mixed tarmac/slush/snow in comparison to steel. Probably easier on the tyres and alloys as you say. For the UK you can't beat a good set of winter tyres for the mixed tarmac/slush/snow conditions we normally experience. You can buy a set of winter tyres+ steel wheels for a similar price to what you might pay for tyres alone here. Complete wheels - Tyres online @ mytyres.co.uk

If you haven't opened the easi grips yet I would try putting them back to Halfords for a credit or refund and go the winter tyre route.

 

[coalville126]

+1 on the winter tyres.

I've got winter tyres on steels for both my W126 and the family MPV. Can't beat them. Snow chains are fine if you are planning on alpine expiditions, but they are too much hassle IMO.

If chains are your thing, check out Lidl next Monday - they are just £19.99 and German TUV, GS marked with a 3-year warranty.

 

[ALNoteman]

Hi Graeme,
Thanks for that web address, I had a look at that and the winter tyres on steel wheels seem quite reasonable, I may give that a go but funds are drying up this month from all the stuff I've bought already and won't stretch to a new set as I do want to upgrade the tyres on the car at the moment for a better quality but maybe I should go for the winters on steel and drive on them till May? In the meantime I could get the alloys refurbished and reshod ready for summer.

Bloody expensive keeping a car in the UK, I also have a 28-ft Motorhome I'm busy upgrading and that is a bit like a boat, standing under the shower ripping Ten Pound notes up, not good for a pensioner?
Al.