The City-El uses special Continental low rolling resistance tires, type 80/70 16 40J (this size designation drives motorcycle dealers nuts, since they use metric designations. There are some moped tires which may fit, bring your rim to the dealer and try it - RRY) developed by the company, which are to be found also on many solar vehicles. The tires have been through three iterations.
Tire changes can be made in the shop or in the field. New tires can be obtained most easily from City-El dealers. In order to change the tires themselves needs one motorcycle tire assembly lever (tire iron - RRY) or the like (bicycle assembly levers are too weak) and a rag or a leather around the assembly levers on the rim to support (the coating of the rim bursts otherwise off). El separate tube has, whereby the tube in the case of a flat with bicycle repairing things could be repaired. When installing the tires one must watch out much that one does not get caught the tube. Something soap or the like helps also when assembling. Without machines its pretty difficult (some curses included). - a motorcycle dealer installed mine for very little money.
One should always have the tire pilot or the like thereby. (Don't leave home without it, the flats on the front are a particularly aggravating pain in the ass - RRY)