Quick question on tire sizing. No definitive answer via search. - The Unofficial BMW M5 Messageboard (m5board.com)

jedigras · 25th January 2006, 03:52

I have done some research regarding winter tires and I have come to the following conclusion regarding 17" rims. Most people prefer the 235/45-17 sizing due to being closest in circumference to the rear OE tires. I also found that people prefer a narrower footprint with a larger sidewall for winter tires so my question is this:

Are people also using the 225/50-17 tires on 17x8 rims? If not, why?

Specification Sidewall Radius Diameter Circumference Revs/Mile Difference
275/35-18 3.8in 12.8in 25.6in 80.4in 788 0.0%
245/40-18 3.9in 12.9in 25.7in 80.8in 784 0.5%
235/45-17 4.2in 12.7in 25.3in 79.6in 796 -1.0%
225/50-17 4.4in 12.9in 25.9in 81.2in 780 1.1%

Hope this makes sense...I may have just confused myself more.

-Brad

DavidS · 25th January 2006, 07:13

What ever rims you get, make sure those will clear the front brake calipers.

To protect the sides of the rims you don´t want a too narrow tyre. Further in some countries (such as Germany) there are very specific regulations about only being allowed to drive with the exact tyre dimensions specified in the car´s registration documents. These dimensions are different for summer and winter tyres for the E39 M5.

What you want from a winter tyre is lateral grip, and grip when braking.
Acceleration grip is of no importance.

My theory on why softer sidewalls are better in a winter tyre is that on the limit when the tyre begins to skid slightly, an tyre with softer side walls will be able to loose lateral traction in a less sharp way than a tyre with hard sidewalls, skip sideways, and then (road surface irregularities permitting)regain lateral grip and will then have more time to softly (elastically) slow down the sideways movement of the car.

(If this theory is correct or not, I don´t know)

David