Winter Tire Sizing for 2007 Z4M

[elerner61]

So here's a question for either our Sponsor (Mr T. Rack) or a know-it-all who trulyknows the answer.

Stock staggered setup from BMW is:
Front: 225/45/18 (yields a sidewall height of 101.25mm)
Rear: 255/40/18 (yields a sidewall height of 102mm)

Front rim dimensions: 18 x 8
Rear rim dimensions: 18 x 9

BMW's specification for winter tire size on the stock rims is the same for front and rear:

Front/rear: 225/40/18 (yields a sidewall height of 90mm)

Oh yeah, the question...why does BMW specify a winter tire size with a lower aspect ratio (40 vs 45) than it's front summer tire? One hypothesis offered is that the lower aspect ratio enables the 225mm wide tire to be mounted on the wider 9 inch rear rim. But this doesn't make sense, since I would think a taller sidewall would better lend itself to mounting on the wider rim.

Second Question: Barring the BMW suggested size, wouldn't it be better to go with the taller tire (225/45/18) that yields a sidewall height closer to the summer tire spec? It would also give better rim protection from winter pot holes.

Thanks!

[elerner61]

Bump. Does anyone know the TireRack rep. that usually lurks this forum? C'mon now, don't make me go hit up Rudy from the Miata.net for this into

[SmartBastard]

Originally Posted by elerner61

So here's a question for either our Sponsor (Mr T. Rack) or a know-it-all who trulyknows the answer.

Stock staggered setup from BMW is:
Front: 225/45/18 (yields a sidewall height of 101.25mm)
Rear: 255/50/18 (yields a sidewall height of 102mm)

Front rim dimensions: 18 x 8
Rear rim dimensions: 18 x 9

BMW's specification for winter tire size on the stock rims is the same for front and rear:

Front/rear: 225/40/18 (yields a sidewall height of 90mm)

Oh yeah, the question...why does BMW specify a winter tire size with a lower aspect ratio (40 vs 45) than it's front summer tire? One hypothesis offered is that the lower aspect ratio enables the 225mm wide tire to be mounted on the wider 9 inch rear rim. But this doesn't make sense, since I would think a taller sidewall would better lend itself to mounting on the wider rim.

Second Question: Barring the BMW suggested size, wouldn't it be better to go with the taller tire (225/45/18) that yields a sidewall height closer to the summer tire spec? It would also give better rim protection from winter pot holes.

Thanks!

the stock rear is 255/40/18 not 255/50/18

Dunno much about what BMW suggest for winter but I'll be running 225/45/18 front and 245/40/18 rear with Michelin Pilot Alpin.

[elerner61]

Originally Posted by SmartBastard

the stock rear is 255/40/18 not 255/50/18

Dunno much about what BMW suggest for winter but I'll be running 225/45/18 front and 245/40/18 rear with Michelin Pilot Alpin.

Thanks for catching my error on the rear aspect ratio. Are you, or have you run that setup? Why not go with the same size if you can for better snow performance with the rears and you could rotate the rears to fronts as they begin to wear (instead of having to replace just the rears earlier).

[ZStig]

I'll be running 225/50/17

[BimmerBri]

Check out my post in the FS area, I have a set of winter Wheel/tire combo FS

[SmartBastard]

Originally Posted by elerner61

Thanks for catching my error on the rear aspect ratio. Are you, or have you run that setup? Why not go with the same size if you can for better snow performance with the rears and you could rotate the rears to fronts as they begin to wear (instead of having to replace just the rears earlier).

I'm going to run the setup this winter for the first time. The reason is, that most of the winter I don't run the car on actual snow but rather wet tarmac (city/highway). I checked on the winter tires and indeed BMW does recommend 225/40/18 for winter, even tho I can't see the logic behind going smaller diameter, especially on the rear where you gonna have to stretch the tire a lot to fit the 9" rim.

[Daemonblitz]

Keep in mind that BMW recommends the 225/40R18 tire size with the Style 164M wheels (not with the stock Style 224M wheels), which are the same width front and back (7.5") and only the offsets are different (ET 41 and ET 20 respectively).

[elerner61]

Originally Posted by Daemonblitz

Keep in mind that BMW recommends the 225/40R18 tire size with the Style 164M wheels (not with the stock Style 224M wheels), which are the same width front and back (7.5") and only the offsets are different (ET 41 and ET 20 respectively).

Have you got a reference doc/site you can point me to for this? These details (164M vs 224M) makes sense, but the only info I have seen is in my manual. Thx.

[Daemonblitz]

Originally Posted by elerner61

Have you got a reference doc/site you can point me to for this? These details (164M vs 224M) makes sense, but the only info I have seen is in my manual. Thx.

The info is from BMWs ETK software. You can basically find the same information on RealOEM.
If you want to run the winter tires on a staggered set of rims you won't have anything to choose from that BMW offers. None of the winter packages currently offered by BMW for any of their cars are mounted on different rim widths front vs back.
If you specifically ask, your dealership would have to custom build a staggered set for you.

If you are ordering the tires yourself I would suggest staying within 3% Revs Per Mile of the stock summer size. (The 164M winter package offered by BMW is off by 3.4% Revs Per Mile from your summer sizes)