My original tyre size is 205/70 /15.  Can I change to 215/50/17 ?

 

Cheers Jim ( 2004 Forester x )

https://www.tacomaworld.com/tirecalc?tires=215-50r17-205-70r15

acording to that your talking a  hit on speddo ready of about 2mph at 70mph, not sure if it will fit direct, tbh might be worth a call to a company like wheelbase who deals with loads of alloys and see if they know, i suspect they will know though.

205/70/15 - sw 143.5

215/65/17- side wall 139.75 (closest to ur original specs)

215/50/17-sw107.5

First, what Tidgy said, second, you'll lower car stands with 72 mm ((143.5 -107.5) x 2) with larger gaps between wheels and car body and reduce ur car ability to absorb road bumps/ cracks/ potholes. 

With this being said, I see no other major problems.