[213e90n51]

been driving on 30psi front and rear square setup for daily. 17x9 225 40. What psi do you guys run for everyday. What do you recommend for daily. I carry no rear passengers.

[bluewater328]

Originally Posted by 213e90n51

been driving on 30psi front and rear square setup for daily. 17x9 225 40. What psi do you guys run for everyday. What do you recommend for daily. I carry no rear passengers.

Recommend whatever BMW has on the inside of your door jam.

[Volasko]

30 Seems low, probably rides decent but once pushed would feel squishy. I always start at 36 psi on all 4 corners and adjust accordingly.

[Tambohamilton]

Worth pointing out that front passengers load the rear wheels about as much as the front wheels... Rear passenger weight is almost entirely on the rear wheels, and trunk luggage actually lifts load off the front wheels. Just food for thought there.

[Efthreeoh]

OP is asking because he switched to a square set up from the staggered set up. My guess. I had the same concerns in 2007.

I switched to square with non-run flats on 8.5" rims. I used 33 PSI all around.

[Gamb1t]

With 225/45/18 on all corners, I have settled on 35-37psi for all tires. I also put chalk marks at the edge of the tire thread/sidewall to determine contact. Old Solo II trick.

[213e90n51]

Originally Posted by Gamb1t

With 225/45/18 on all corners, I have settled on 35-37psi for all tires. I also put chalk marks at the edge of the tire thread/sidewall to determine contact. Old Solo II trick.

no wonder in autoX i see them use chalk. But for daily driving can i put chalk to?

[ewicky]

30 does seem a little low for daily driver, but that could be fine for your use case.
I'm sure BMW has put a lot of thought into their recommendations. The problem I have with trusting that, is that they might not have prioritized the operating characteristics in the same way each driver would.

For example, tire pressure variables include:
-Desired fuel economy
-Tire size
-Desired tread wear
-Curb weight vs full gross
-Cornering handling
-Straight-line traction
-Resistance to hydroplaning
-Noise (and NVH in general)
-Influence on Understeer/Oversteer tendency
-Steering feel
-Tram-lining tendency
-Heat buildup at high speeds
-Tire construction, such as run-flat vs regular
-Enough overhead that a tire that hasn't been checked for months and the weather has gotten cold still has good enough pressure.

BMW is spec'ing their pressures for more weight than just me in my car, higher speeds than I'm allowed to do in the US, and RFT factory tires, as well as overhead. These don't align with my use case.

[The Nightman]

Tire pressures depend on the load index of the tire in question. The proper way to determine the pressure of your new tires is to work backwards from the manual using BMW's specified tire load index and pressure to find the load per tire in lbs. Then look up the load index of your new tire to determine which pressure best matches (equal or greater than) the load per tire in lbs. Note that there are different load inflation tables for standard (SL) and reinforced (XL) tires.

https://www.toyotires.ca/sites/defau...s_20170203.pdf

So based on the load index and pressures specified in the 2009 328i manual for sport package models and 17" square and staggered tires:

Square:
225/45R17 94V XL @ 32 PSI: 1179 lbs (46%)
225/45R17 94V XL @ 39 PSI: 1400 lbs (54%)

Staggered:
225/45R17 91V SL @ 32 PSI: 1226 lbs (46%)
255/40R17 94V SL @ 35 PSI: 1433 lbs (54%)

The new tire pressure for my square 235/45R17 97W XL tires would be:
235/45R17 97W XL @ 30 PSI: 1246 lbs (47%)
235/45R17 97W XL @ 36 PSI: 1433 lbs (53%)

[zumone]

Originally Posted by Guy Fieri

Tire pressures depend on the load index of the tire in question. The proper way to determine the pressure of your new tires is to work backwards from the manual using BMW's specified tire load index and pressure to find the load per tire in lbs. Then look up the load index of your new tire to determine which pressure best matches (equal or greater than) the load per tire in lbs. Note that there are different load inflation tables for standard (SL) and reinforced (XL) tires.

https://www.toyotires.ca/sites/defau...s_20170203.pdf

So based on the load index and pressures specified in the 2009 328i manual for sport package models and 17" square and staggered tires:

Square:
225/45R17 94V XL @ 32 PSI: 1179 lbs (46%)
225/45R17 94V XL @ 39 PSI: 1400 lbs (54%)

Staggered:
225/45R17 91V SL @ 32 PSI: 1226 lbs (46%)
255/40R17 94V SL @ 35 PSI: 1433 lbs (54%)

The new tire pressure for my square 235/45R17 97W XL tires would be:
235/45R17 97W XL @ 30 PSI: 1246 lbs (47%)
235/45R17 97W XL @ 36 PSI: 1433 lbs (53%)

Thank you for this info,

I used it to calculate ideal pressures for my 225/40R18 92Y (front) and 245/40R18 97Y (rear) but ended up with lower pressure at the back than the front when fully laden, does this seem correct? Seems counter intuitive

[The Nightman]

Originally Posted by zumone

Thank you for this info,

I used it to calculate ideal pressures for my 225/40R18 92Y (front) and 245/40R18 97Y (rear) but ended up with lower pressure at the back than the front when fully laden, does this seem correct? Seems counter intuitive

I haven't checked your numbers but it should be fine if those loads are accurate. It basically means the 97 load rated tire is capable of supporting that weight at a lower pressure.

[zumone]

Originally Posted by Guy Fieri

I haven't checked your numbers but it should be fine if those loads are accurate. It basically means the 97 load rated tire is capable of supporting that weight at a lower pressure.

Alright that makes sense, thank you.

[zumone]

Originally Posted by Guy Fieri

I haven't checked your numbers but it should be fine if those loads are accurate. It basically means the 97 load rated tire is capable of supporting that weight at a lower pressure.

I checked using this calculator

https://tiresize.com/pressure-calculator/

And obtained the same exact values, so I sure they are correct. Thanks for your help