Tire pressures for larger sizes

[E39Fiver]

I have just installed new wheels/tires on my '01 530 non-sport and was wondering if anyone had imput on correct pressures for the new sizes. The original tires were 225/55R16 all around and the factory recommendation was 33F and 41R.

I have gone with 18X8.5F and 18X9.5R wheels with 245/40R18 and 275/35R18 tires. The question is mainly concerning the rears, as the fronts "look" about right at 32. The rears seem like they are WAY overinflated at 40 so I have lowered them to 34 and they "look" better but I wonder if anyone (Gary?) could provide some input on their experience?

Thanks in advance -

[E39Fiver]

- BUMP -

Anyone?

[jvr826]

IMO, 32psi is too low. I ran 38-40psi per corner when I ran 18's on my 530i, but that was indicated by the tires I used. They happen to like more pressure. I run about 36psi per corner on the 18's I run on this car, about the same on the 17's I use at the track. General rule I was told for track driving, don't exceed 40psi hot.

Tire size doesn't matter. Just cuz they're bigger, doesn't mean you need more or less air. It all depends on wear, handling, ride, whatever works for you. But, running too low you'll have potential for sidewall failure. I generally start at 35 and work my way up from there.

[E39Fiver]

Thanks for the reply, Jeff -

I am at odds with your claim that tire size doesn't matter, though - Compact spares run higher pressures (sround 60+ psi) because of the smaller volume, the huge floatation tires on my tractors at work only carry 8 psi, and my bicycle tires (an extreme example, yes) run 110 psi. It seems to me I have increased the volume of the rears by quite a bit, and am carrying the same load, so the psi should drop accordingly.

With the stock negative camber in the rear, the 275/35R18 Kumho ecsta ASXs have the outside shoulder blocks off the ground at anything over 32 psi. They do roll into full contact under hard cornering, but not having a starting point I was wondering...

FWIW, I almost never have more than just me in the car and I did wind up adjusting pressures from stock with the 225/55R16s to 36F 38R. This setup feels different, but I have yet to push it hard enough to find the handling limits.

Thanks again!

[Gary@Tirerack]

Keep in mind that your 18" tires actually aren't any 'bigger' in diameter or in terms of load capacity or total air volume. JVR826 is mostly right for in this case that your 18" setup you use the same psi as OE regardless of how the tires 'look'.