Space saver should always be put on the back though. You shouldn't have 2 different sized wheels on the front as this is where the power is going and the wheels you use to turnNot sure but I know the fn2 space saver only fits either the front or rear doesn't fit both so if you get a puncture in one that it doesn't fit you have to take another wheel off swap it with the punctured one and then put the spacesaver on the one you took off if that makes sense.