Originally Posted by
FastJake
If you use a 20mm tube in a 40mm tire it will be too narrow and won't inflate the entire tire, causing really bad ride and handling.
A 20mm tube will have no trouble at all expanding large enough to fill a 40mm or even larger tire. It'll be stretched rather thin and will therefore be more permeable to air than usual (i.e. require more frequent pumping to maintain full pressure).
Note to OP: tire widths are given in mm, there's no reason to put a "c" after them.