The latest hi-tech zenith of efficiency is the Viscotronic fan. This has an engine management controlled variable speed viscous fan. It is an amazing piece of kit. I happen to have a bit of experience with an application where a diagnostic laptop can be linked to the vehicle and the speed of the fan can be infinitely varied within reason by means of mouse on slider. It so happens that I have two near identical vehicles except that one has a simple viscous coupled fan and the other having the latest viscotronic unit and the reduction of noise from the latter is very noticeable. With the same fan blades it is probable that this equates to a significant gaining of horsepower or reduction in fuel consumption.
Huw