[mojoJJ]

I recently switched out stock RFs with taller tires on my LCI G06 40i. In the iDrive settings, I’ve had to select “all year” and “other” tires and it no longer shows recommended pressure.

I used tire size calculator site to calculate new pressure but need help making sure whether it’s correct or not.

OEM tires - Pirelli PZ RF
275/40/21 XL recommended pressure 35 PSI
315/35/21 XL recommended pressure 38 PSI

New tires - Michelin Primacy Tour AS
275/45/21 SL
315/40/21 SL

[X6ix GTA]

I'm 99% sure you need to use the stock tire pressure. Upsizing or downsizing the tires doesn't change the recommended pressure as far as I know.

[mojoJJ]

Originally Posted by X6ix GTA

I'm 99% sure you need to use the stock tire pressure. Upsizing or downsizing the tires doesn't change the recommended pressure as far as I know.

Well, technically it should, since it changes the aspect ratio and the load bearing capacity. I’m just not sure what it should be. I think there’s a way to look up axel weight and look up the tire chart and calculate.

[roamio]

Originally Posted by X6ix GTA

I'm 99% sure you need to use the stock tire pressure. Upsizing or downsizing the tires doesn't change the recommended pressure as far as I know.

I think it's wrong - tire pressure will and should change with different tire sizes. Simply open user guide and compare tire pressures for 20", 21" and 22" sizes. Higher sidewall - less tire pressure. My rear 22" according to BMW is 48 PSI. Same car with 20" is 42 PSI.

[eelnoraa]

Originally Posted by roamio

I think it's wrong - tire pressure will and should change with different tire sizes. Simply open user guide and compare tire pressures for 20", 21" and 22" sizes. Higher sidewall - less tire pressure. My rear 22" according to BMW is 48 PSI. Same car with 20" is 42 PSI.

I agree. Start with the pressure they supports the weight. You have to experiment a bit to determine optimal pressure if change away from stock. Typically, as side wall increase pressure decrease. The very low profile tire need very high pressure.

[ScaldedDog]

To be slightly nit-picky, it's the change in volume that affects whether or not a different pressure is required, or necessary. All else equal, a larger volume tire will require less air pressure to support the same load. The difference can be calculated, but you end up having to play with it, anyway, so you may as well just start there.

Mark

[rexus300]

Since the tire size has gone up so does the volume of air so the pressure will be lower per square inch to support weight of the car. The load capacity is lower with the standard load tire (SL) vs the oem extra load (XL) so you will be fine at 35 psi front and 35-38 psi rear depending on normal or max load.

[X6ix GTA]

Originally Posted by roamio

I think it's wrong - tire pressure will and should change with different tire sizes. Simply open user guide and compare tire pressures for 20", 21" and 22" sizes. Higher sidewall - less tire pressure. My rear 22" according to BMW is 48 PSI. Same car with 20" is 42 PSI.

Thanks for correcting me. I wasn't aware. Will read up more on this topic.

Sorry OP, didn't mean to mislead you. Just shared what I've been practicing.

[aaaaaaaaaz]

Umm you are effectively changing the diameter of the tire, is there any recalibration needed? Otherwise speedometer might be off.

I think generally, the minimum psi is usually decided by the weight it supports; while the maximum psi is decided by the tire it self (usually around 60).

Since you are increasing tire volume (sidewall) the recommended 35/38 would be safe; and due to your tire being taller you can run a bit lower, but I'd start with that value. Generally it is ok to run a few psi above recommended value, but the other way around might cause issue.

[DaveDBD]

Just FYI. The Pirellis are both 701 rpm, the Michelin are 676 front and 673 rear. So you now have speedometer and odometer both low about 4%.

The Pirellis max load is at 50 psi, the Michelin at 44 psi. Any BMW tire pressure specs will be based on XL tires and should not be used with SL tires.

The tire pressures the website recommended look correct, but maybe add 2 psi to begin with. Better to run higher pressures than lower to reduce heatup.

[jadatis]

Googled the OEM and new tires, and had same loadindex, but SL instead of XL
So sameaxload, but XL at 42 psi and SL at 36 psi.
Front loadindex 107 and rear 111
So the calculation is right of the new pressures.

But better would be to let me calculate pressures for GAWR's, or better real axleweights, and maximum used speed.

[TexasBimmer89]

Originally Posted by mojoJJ

I recently switched out stock RFs with taller tires on my LCI G06 40i. In the iDrive settings, I’ve had to select “all year” and “other” tires and it no longer shows recommended pressure.

I used tire size calculator site to calculate new pressure but need help making sure whether it’s correct or not.

OEM tires - Pirelli PZ RF
275/40/21 XL recommended pressure 35 PSI
315/35/21 XL recommended pressure 38 PSI

New tires - Michelin Primacy Tour AS
275/45/21 SL
315/40/21 SL

Going to follow this thread as I will be making this switch soon as well.

Thanks OP.

[eelnoraa]

I don’t get why you guys want to change away from oem overall diameter. 4% is although not a whole lot, it is not smalle either. car will drive differently. It is effectively changing gear ratio by 4% for every gear.

[TexasBimmer89]

Originally Posted by eelnoraa

I don’t get why you guys want to change away from oem overall diameter. 4% is although not a whole lot, it is not smalle either. car will drive differently. It is effectively changing gear ratio by 4% for every gear.

For me, at least, it’s so I can drive my car all year long (and not park it during the winter) and tires that last longer than 16k-18k miles.

[mojoJJ]

Originally Posted by eelnoraa

I don’t get why you guys want to change away from oem overall diameter. 4% is although not a whole lot, it is not smalle either. car will drive differently. It is effectively changing gear ratio by 4% for every gear.

I’ve now driven ~1000 miles without any problems.