ok , i think i am going to go non runflat, the next question is the size of wheels/tyres on my x5 are

front 275/40/20
rear 315/35/20

i can pick up a good second hand spare alloy and tyre but what size would i need to go for to be compatible ?
this will be cheaper than buying a space saver from BMW , it will only be used as a spare and for a short period of time in the event of a puncture, i am just unsure what size that i need to go for

thanks
gary