What is the most efficient way to increase a radiators efficiency?

To increase a radiator’s efficiency the most efficient way that is used.

The temperature difference between the coolant and air passing through.

The function of a radiator is to transfer heat from the engine to the atmosphere or the air passing through it. There are two types of radiators depending on the flow of coolant inside the radiator, viz. down flow and crossflow radiators. The efficiency of the radiator depends on a number of factors: the area of contact, the temperature difference, and the coolant capacity. The area of the radiator depends on its design and compactness of the engine whereas the difference in temperature depends upon the coolant inside the radiator and the temperature of the cooling air passing through it. The efficiency also depends on the pumping power as higher pumping force to reduce the coolant temperature faster, but it also needs higher energy to run the pump.

Conclusion:

As the area and other design parameters of a radiator depend on various aspects, the efficiency of the radiator depends on the temperature difference between the coolant and air passing through.