my 850 is fitted with 205/55/15 tyres anybody know whats the biggest 15" tyre i can fit 215,225?

 

The 215 is ok but the 225 would be pushing it.

 

My 850 currently has 205/50/R17 and the front passenger's side tire has worn thru plastic inner wheel fendor.

Using this formula, it has tire diameter of 25.07 inches

Height = (Width x (Ratio/100) x .03937 x 2) + Rim Diameter
(205 x (50/100) x .03937 x 2) + 17 = 8.07085 + 17 = 25.07085

215/60/R15 would have tire diameter of 25.17 inches
(215 x (60/100) x .03937 x 2) + 15 = 10.15746 + 15 = 25.15746

195/65/R15, the original size for 15 in wheels, has tire diameter of 25 inches
(195 x (65/100) x .03937 x 2) + 15 = 9.980295 + 15 = 24.980295

205/55/R16, the original size for 16 in wheels, has tire diameter of 24.9 inches
(205 x (55/100) x .03937 x 2) + 16 = 8.877935 + 16 = 24.877935