Uaglio is absolutely correct. If you are using original wheels you must use a tube in your tyre. A modern wheel has a 'lip' built into it which holds the bead of a tubeless tyre in place---the original Fiat 500 wheels do not have that 'lip', hence the requirement of the tube. The original tyre size was:--125 x 12, but you can safely go up to 135 x 12, which puts just a little more rubber on the road, and VERY slightly increase your final-drive ratio--engine will turn slightly slower for the same speed, but watch for a slight difference in speed accuracy.