I currently have 17's fitted with 205/40/17 but want to go 15" so I was thinking 195/50/15 will keep the speedo about right but can I go 195/50/15 on the front and 205/50/15 on the rear? would there be any advantage??

also the current ET is 38 and the new wheels I'm looking at are ET30 so my reckoning is that the wheels will stick out appx 8mm, not a problem on the rear as they have 5mm spacers at present which I can get rid of and if the front stick out too far then I can always re drill the holes on the cycle wing holders to make sure they are covered.

Am I going nuts or is the above making sense?

Makes sense
Difference is -1.68%
Or -10mm
No big shakes

[Edited on 16/9/13 by snapper]

cant see a problem i run 225/50/15 on rear and 205/50/15 on front mind would not go any bigger on front

This link will give you a good idea about fitment.
http://www.willtheyfit.com/

thank you all

That web link is very helpfull