[Thermobaric]

I have PA3 235/40/18 95V XL on each corner as my winter tyres. As these are different to the OEM tyre size, I just wanted to check correct pressure. As they are closer to OEM 225/40/18, should I be using 240 all around? And not 260 at the rear?

[Digitalize]

Tyre pressure has nothing to do with tyre size. It's pressure not volume. The pressure's are more to do with the weight over the wheels/weight rating of the tyre/handling BMW want than tyre size.

Use the recommended pressures for front and rear, and go from there. All the changes are very small anyway so just use the regular as a starting point, and change them depending on the feel you want, or if you're getting irregular tyre wear.

[jadatis]

The 225/40/18 have loadindex of 92 in XL /reinforced/Extraload.
So if XL OEM you could use lower pressure then adviced for OEM tires because going from LI 92/ 630kg maxload to LI 95/690kg.

Weights on the wheels stay the same.

And both OEM and new bare the maximum load AT 290kPa/42psi up to 160km/99m/h. For higher speed there is a system for highening up this AT-pressure but who drives faster?

But I am able to calculate it for you if you can give weights.
Can also calculate it for different settings, like fully loaded or normal use.
For all I need to know best determined weights on wheels or axles.

Car maker uses nowadays Max permissable vehicle weight given on VIN-plate. But if you can give empty weight and the way you load it normally ( Persons and load) I can do that estimation for you .
Determining the weights is the most tricky part in this all.

Your kind of low H/W tires is another point of interest, best is to lower the maximum load by 8 LI steps.

Greatings from a Dutch Pigheaded Selfdeclared Tyrepressure-specialist.
Peter

[Thermobaric]

Thanks Digitalize, Peter.

That's good to know. I was just maintaining them at the 240/260 as supplied by my fitter. It makes sense it has nothing to do with volume. I hadn't really thought about that and just had a random thought after looking at the door card heh.

As it only has me/1 passenger with no/minimal luggage 99% of the time, I keep them at the lower end of load weight. Plus I don't plan on going 186MPH so don't need the lower pressure for Y rated tyres.

[Digitalize]

The pressure in relation to load/speed rating is to do with the construction of the tyre, not the actual speed/weight they'll experience. They are generally a stiffer tyre, so a lower pressure may be recommended to maintain a similar feel.