I just purchased some slightly used BF Goodrich G-Force KDW 2s in 215 / 40 / 17 to put on my R90 17" wheels. Will there be any fitment problems? My car is completely stock ('06 JCW MINI Cooper S) with stock suspension.

I know the stock wheel size is 205/45/17, which is why i'm asking. What are the disadvantages of putting on a wider tire?

 

Should work fine. I know folks running those tires in 215/45/17 size with no problems.

 

Actually, I think the 215/40/17 is a better fit than the 215/45/17...I'm running the Kumho MX 214/40/17 on my MCS...My speedo reads 2 MPH faster than I'm actually going, according to my GPS.

 

Better in what sense?

 

In your case it will fit fine.

205/45-17 fits rims 6.5-7.5" wide, perfect fit for 7" rim, 24.3" tire diameter
215/40-17 fits rims 7-8.5"wide, perfect fit for 7.5" rim, 23.7" tire diameter

For a 17x7" rim both tires fit. 205/45-17 is a perfect fit. the 215/40-17 is slightly smaller than stock tire diameter so expect it to ride a little harsher and you'll get some speedo and odometer error. Wheel gap will be increased since you have stock suspension.