I have a 2005Ford Focus it has 16 inch rims on it I
would like to use 15 inch rim on it for spare would it
cause rubbing or any other problems .
It has to be the same offset as what you have now and the tire diameter needs to be the same. If you do get a flat on one of the drive wheels I recommend moving a rear tire forward and putting the spare on the rear, that will elemanate any problems you might encounter with the spider gears inside the transaxle.
Thanks that's what I will do then