Why are the best glide and best range airspeeds different for a C172?
Maximum range depends not only on the variation of lift to drag versus airspeed, but also on the efficiency of the engine and propeller versus airspeed. The C172's fixed pitch prop, fixed ignition spark advance timing, fixed valve lift and closure timing and other characteristics all have effects on the miles per gallon attained at different indicated airspeeds along with the lift to drag of the airframe.