[tranquility]

Originally Posted by chad86tsi

Is safety measured by :not blowing out tires, or slipping from uneven wear from under inflation, or : not slipping in a corner from stiff sidewalls due to over inflation? Your single criteria is not going to yield a single best answer. We aren't being difficult, it just a complex relationship.

Safety in terms of having the best grip/tire being most effective above all else. It really shouldn't be all that complicated esp when we're talking about winters. For example, I don't mind at all and in fact prefer that they are RFTs, because the performance aspect is thrown out the window for me when it comes to winters.

[chad86tsi]

Originally Posted by tranquility

Safety in terms of having the best grip/tire being most effective above all else. It really shouldn't be all that complicated esp when we're talking about winters. For example, I don't mind at all and in fact prefer that they are RFTs, because the performance aspect is thrown out the window for me when it comes to winters.

Run lower pressure, you'll get a softer more compliant side wall which helps with shock-load lateral traction-slippage (no idea what else to call it). It will also increase contact patch.

The trade off is it will shorten the life of the tire, and will make handling soft and spongey by comparison. It may also get louder road noise, but may not, there are other factors in play on that one.

If the specs call for split pressures F/R, favor the lower one. Use a simple ratio formula on their width to check your target numbers.

[tranquility]

I guess I'll have to do more testing. I drive pretty conservatively in winter, so I don't think my changing the psi will make too much tangible difference since I'm within the acceptable range anyway and knock wood I've always managed to control my car even when I hit a black patch of ice, etc.

[chris719]

Originally Posted by tranquility

I dunno, that's really low from what my manual says (but, unfortunately, it only talks about staggered, so F36 and R42).

I thought the table someone pasted earlier showed lower pressures, the 36/42 was for the >100 MPH or heavy load scenario?

Anyway, my recommendation is based on the fact that I owned an E85 Z4M roadster, which weighs slightly more than a 3.0si I think, and it recommends 32/32 F/R on the door jamb stock. I ran 225 square winter tires for years like that. Can't say I tried playing around with it, though. BMWs that come with RFT tires seem to have high recommended pressures, so if you are using a non-RFT winter tire I might be tempted to use the M pressure since it comes with non RFT tires.

[tranquility]

Originally Posted by chris719

I thought the table someone pasted earlier showed lower pressures, the 36/42 was for the >100 MPH or heavy load scenario?

Anyway, my recommendation is based on the fact that I owned an E85 Z4M roadster, which weighs slightly more than a 3.0si I think, and it recommends 32/32 F/R on the door jamb stock. I ran 225 square winter tires for years like that. Can't say I tried playing around with it, though. BMWs that come with RFT tires seem to have high recommended pressures, so if you are using a non-RFT winter tire I might be tempted to use the M pressure since it comes with non RFT tires.

Ya, but please remember that my E89 is a porker compared to your E85, so I assume that the higher PSI is warranted lol.

I recall from my manual that the F36/R42 was for reg speeds not just >100mph but anyway it doesn't really apply here because they are specific to a staggered setup of 225/255 18" and my winters are a square RFT 225 17".

[chris719]

Originally Posted by tranquility

Ya, but please remember that my E89 is a porker compared to your E85, so I assume that the higher PSI is warranted lol.

I recall from my manual that the F36/R42 was for reg speeds not just >100mph but anyway it doesn't really apply here because they are specific to a staggered setup of 225/255 18" and my winters are a square RFT 225 17".

My bad, I didn’t realize it was an E89. Disregard my advice on that then.

[tranquility]

Originally Posted by chris719

My bad, I didn’t realize it was an E89. Disregard my advice on that then.

No sweat, if only I had an E86 then I wouldn't have this prb

Originally Posted by Piper1

I just looked up on tirepressure.com and states for the e89 ‘09-10 225/45/17 square 32F 35R ‘11+ is 36F 44R so start with that see what happens. You might find you need to add to front reduce the rear or add on both or reduce both I doubt you’ll reduce the fronts but that will depend on how you see the tires wear.
https://tirepressure.com/bmw-z4-tire-pressure

Tx for researching, I'll have to experiment and report back.