Brakes are non-brembo. Calipers and rotors are much larger than the 03 and the 03's spare tire hits the caliper.

Will this spare tire clear the 06 brakes? (It's much cheaper than the 06 oem spares)

http://www.ebay.com/itm/INFINITI-G37...74fb8d&vxp=mtr

 

Odd, it shouldn't hit. The caliper doesn't stick outboard any more than the stock. Because it's it's further away from the center of the rotor? Getting another spare would be the right move. I'd think a bone yard would have them way cheap and local.

Or if you get a front flat, move the rear to the front and put the spare on the rear until you get a proper spare

 

That's what happened to me the other day. I tried to fit the spare and it wouldn't fit. I had to put the flat tire back on, jack up the rear, take the rear tire off, then put the spare on the rear. Finally I moved the rear tire to the front. Ugh. I guess I'll visit some junkyards this weekend.