Why Should I Replace My Tubes?  Who Says So?

 
Reuse Of Tubes
  A new tube should be used when installing a new tire.  Tubes grow in service, taking a permanent set of about 25% larger than the original size.  This makes a used tube too large to use in a new tire, which could cause a wrinkle and lead to tube failure. 
Source:  Goodyear Tire Care & Maintenance Guide 09/98


Used Aircraft Tubes
  Aircraft tubes are made of natural rubber and are made slightly undersize, so they will fit easily into a new tire.  Aircraft-tire plies are made of nylon, and will "grow" slightly in service.  The tube will also grow, and will eventually take a permanent set to the (now) larger inside-tire dimension.  If this now-larger tube is later put into a new tire, it may be too large for the inside-tire cavity, with the result that the tube may have folds in it.  In service, these folds may eventually wear through and destroy the tube's ability to hold air.  A slow wearing through will result in a slow leakage of air, and the pilot will be alerted to the problem before it becomes too serious.  However, if the tube is torn in a particular takeoff, the tire will probably go flat in mid-air, without the pilot having realized what is wrong.  Considering the risks in reinstalling a used innertube, we recommend a new tube installed in a new tire. 
Source:  Specialty Tire Product Guide