Re: M5 Snow tires--size help (archive)

But to answer your question: you have to try and match the mounted tire's diameter. You said stock front is 245/40-18. So 245mm/25.4= 9.65in x .40 = 3.86in x 2 (top and bottom)= 7.72in + 18in (for the rim)= 25.72in...

Now if you do the same for the two options you suggested:
245/45-18 = 26.68in
255/40-18 = 26.03in

Clearly the 255/40-18 is closer to the stock 25.72in. I have heard that you should try to keep within 1/2in, so i'd stay away from 245/45-18 or you might have problems in the wheel-well especially on the fronts.

I should also mention that snow tires are often more effective in narrower sizes. So if you must squeeze winter tires on your normal rims, you might consider 235/40-18, 225/45-18.

For the rears, I'd look at 245/40-18, 255/40-18, 265/35-18, 285/35-18. Again with a preference for something narrower.

Some other things to consider,
if you do get a wheel package (which will be a lot easier to switch on-off as the seasons change) don't get small rims 'cause i think you'll have trouble with those big brakes!

the tire-pressure monitoring system will probably need to be reset.

Good luck - mine arrives in 3weeks

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I found what I think will be acceptable winter tires for my M5. The Pirelli SnowSport tires come in 18" sizes that should work well on the M5. However, they do not come in exactly the same sizes as the OEM tires. I know that I cannot compromise on the diameter (18"), but do I need to have the EXACT same sizes, or can I use slightly different ones. For example, the front tires are 245/40ZR-18, but the closest Pirelli is 245/45HR18� or 255/40VR18. The rear tires are 275/35ZR-18, and the closest Pirelli is 265/35VR18.

Is this going to work? And exactly what are the rules of interchanging tire sizes?

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