R134a is NOT less efficient than R12 – Actually R134a is more efficient.
Pound for pound R134a is a more efficient refrigerant than R12, however it runs at higher pressures in some aspects and therefore requires more effective condensing. Whether R134a performs as well as R12 in any given a/c system depends upon system components and the amount of R134a used.
Given two identical vehicles, each with the same weighted amount of refrigerant, the vehicle with the R134a has the capability to remove more heat (measured in btu’s) from the vehicle than the same type of vehicle using the same amount of R12.
The most common influences which effect the capability of R134a to perform well are the condenser, in some cases the superheat setting of the expansion valve or the amount of R134a. Condensers designed to release greater amounts of heat help to expel the greater amount of heat which R134a removes from the car’s interior. And by matching the correct amount of R134a to use in a given vehicle, correcting the superheat of the expansion valve (if necessary), you can in some manner nearly balance or match the amount of heat drawn out by the evaporator and released by the condenser.
These efforts to balance the system can not be realized if there are problems within the a/c system, such as: poor performing compressor, dirty condenser or poor air flow through the condenser, malfunctioning expansion valve, water or air in the system, improperly operating fresh air or heat input in the climate-air mixing system.