[luke1990]

Hi guys,

Peculiar - and maybe very simple question . I asked on G30 forum, but no reply as traffic is low there. My question is theoretical in nature - so same would apply here on F series cars. I'm going from 245/40 275/35 R19 at recommended 2.2/2.6 bar to 255/40 and 295/35 R19. Since the tire size changes, I assume I need to adjust the pressure accordingly. Could you please check my understanding - if its correct?

The way I see it its about the sidewall. We know how BMW scales it:

245/40 + 275/35 R19 has 2.2/2.6 bar recommended
245/35 + 275/30 R20 has 2.5/3.0 bar recommended

245s going from R19 to R20: -12.8% sidewall and +13.6% pressure (2.5 vs 2.2)
275s going from R19 to R20: -15.8% sidewall and +15.4% pressure (3.0 vs 2.6)

This means that the pressure adjustment is very proportional to the sidewall change. I would think the difference comes from the fact that you round the pressure.

Hence with my setup:
255 vs 245s: +2.6% sidewall so run 2.1 instead of 2.2 for -4.5% pressure or 2.15 for -2.3% presure
295 vs 275s: +7.9% sidewall so run 2.4 instead of 2.6 for -7.7% pressure

Resulting setup on 255/40 + 295/35 R19: 2.15/2.4 bar. Am I right? Thanks! I think my calc makes sense but would like a confirmation

[HighlandPete]

Why not simply try the same pressures as the OEM, and adjust according to driving feel and use.

Load being carried is the same, so balance of ride/handling vs. wear patterns will really govern where you want to sit for ideal pressure.

Remember BMW take more into account, that simply the tyre size and load index. Recommended pressure settings include assessment of the best driving experience/compromise.

Anyone using tyre sizes outside BMW OEM specs are really 'experimenting' and doing there own development work, to arrive at the best setting pressures.

Things to also consider: Tyre flexing, you'll have more, heat generated, may need extra pressure to control heat. Type of tyre, if adding XL to the mix, pressures vs. load have to be considered. Your particular use is probably the key factor, to what you set the pressures at.

I'd experiment, (like I do with OEM sizes), you soon get to know where pressure should be for a decent drive and tyre longevity.

[J555]

Originally Posted by HighlandPete

I'd experiment, (like I do with OEM sizes), you soon get to know where pressure should be for a decent drive and tyre longevity.

Exactly!

[luke1990]

Well I would like to base my experiments on something . The tire will wear poorly if pressure is too high or too low. This I will not feel myself, but it is detrimental. Further it will impact driving - too much pressure and grip is reduced.

So ideally I would like to understand where I should start

[J555]

I think you are perfectly fine to start, where your calculation leads you... If you would feel the ride is soft for your taste, bending sidewalls or even see the wear on them, increase the pressure by 0.2. Don't worry, if you are checking your tires regularly, and avoid both low (<2 bar) and very high (>3 bar) pressures, you should be perfectly fine.

In the link you are referring to, they define the same tire pressures for both 245/40/19 and 245/45/18; regardless of tire brand/model used... Speaking of sidewall, important is also how stiff the sidewalls of your tires are, not just their height. A significant differences there, not going to comment run-flats (hope not your case).

Also important, what is your driving style and how loaded the car usually is. If you are aggressive on the country roads, going really fast on the highways or having the car fully occupied, you should comfortably go up by 0.2-0.3 bar above your standard compromise. On the other hand, for slow city driving, no problem to go slightly down for better comfort.

While I don't know what tires you have, my guess for the best compromise would be 2.2-2.4 front, 2.4-2.6 rear. If you do care about the wear and fuel consumption, the higher pressures certainly help. However, a very high pressures can not only make the ride harsh, but also cause an uneven wear in the center area (the other unwanted extreme from the apartment wear on sidewalls).

Don't forget to recheck your geometry; something, that can ruin all your tire pressures care.