1970-73 Challenger rear Backing plates

E-body will be 10x2.5”, 11x2.5” or possibly even 11x2”.

Just measure the outer diameter of the plates, they’re 1” larger than whatever the brake drum diameter is. So the 10x2.5” brake backing plates will be 11” for the outside diameter, the 11x2.5” (or 2”) will be 12” outside diameter.