i'm pretty much in the same boat, how big a side wall will the 16 need  to match the 17 with 45 profile.
from what i know driving with diff size tyres will stuff the diff

With AWD all tyres have to be within a certain percentage of the diameter otherwise damage may be caused to drive systems. I'm not sure of the exact parcentage for Subaru but it could be around 5% of the the rolling radius. To be sure if your 16" tyre will be OK use this tyre size calculator　

d_generate has hit it right on the head.. and even if it is off sized, its a spare tyre.. meaning only temporary to get you home.. pro-longed driving with different sized wheels will do damage.

Hey Alex, I take it that you only have the standard 16" steely as the spare aswell.

But as Brendan said the spare is only to limp the car home so that the tyre can be taken to a tyre joint to be fixed or replaced

alexxx;129813 wrote:

haha thats nothing! if i get a flat on the front of my car i will need to jack up the whole side, swap the back wheel to the front and change the back due to my brakes being to big for spare!

Thanks guys.

Alex, that's a very good point you just raised. If I upgrade to a 294mm rotor on the front, I doubt that the steel 16" spare wheel will clear the calipers! I'll be in the same situation as you in swapping a rear alloy to the front and putting the spare on the back!

Might try and track down a sh!tty 17" alloy wheel for the spare.

sarbarooliberty;129836 wrote:

Hey Alex, I take it that you only have the standard 16" steely as the spare aswell.
But as Brendan said the spare is only to limp the car home so that the tyre can be taken to a tyre joint to be fixed or replaced

i'd say the 16inch wheel would fit as the fozza from 01 onwards had these brakes standard (on all models 294mm). i only have 15inch steel wheel spare as my RX came with 15inch alloys.