Is an Electric fan conversion worth it? My testing procedure
 

So I have been thinking about converting to an electric fan to try and save some fuel. The mechanical fan seems to pull a lot of air even when the truck is cold/fan clutch is disengaged. I have never noticed the fan cutting in except for when towing or climbing big hills during the summer, so the electric fan would not have to run very often.

I wanted to test how much drag the mechanical fan actually puts on the engine and was hoping someone could verify that my test makes sense...

I plugged in my code reader which has a live data feature that can display Load %.

With the mechanical fan still in place I drove a loop and recorded the Load % value on a flat stretch of road at 60 km/h to be a 10.5%, and on a long hill at 50km/h the load read 25.0%.

Then I removed the mechanical fan and drove along the same route, and on the same flat section of road the Load was a steady 9.0% and on the same hill as before it was 22.0%.
The temp gage stayed between 80-90 Celsius during both tests.

So thats a 14% reduction in load on the flat, and a 12% reduction on the hill?? It surprises me much load could be from the fan alone. From my test results I think I'm going to go ahead with an e-fan, it seems like I should see some noticeable fuel economy gains, should be close to a 4% increase in mpg if you figure the engine converts 1/3 of the fuel to mechanical energy (as long as my math is correct and 1/3 may be a bit optimistic I'm not sure).

I have heard that if you take the fan off your water pump can wear out quicker as it is used to having the fan pull on the shaft/bearings, I've also heard that it will last longer because you don't have the fan hanging off of it? If anyone has done an e-fan conversion please let me know if you've had or heard of any water pump issues.

To me you should have done another test after putting the fan back on just to make sure that it wasn't something like your fuel being warmer (return line from the engine) or tires being warmed up.
but all in all, your fan is running at crank shaft speed, right? if you put that fan on an electric motor and ran it at the same speed how many watts would it pull?
Our truck at work is used as a truck towing near it's own weight half the time, so from that point of view it's most likely using the cooling of the fan while it's running and converting mechanical energy to electrical and back to mechanical seems like a round about way to get work done, but for your use it seems like it's not needed very much.

I like the idea of putting the fan on an electric motor. Then would you just use P=IV? but how would you know what rpms its turning? and what is the average efficiency of an electric motor? 90%ish?

On a technical note, as Ryland said, an e-fan is "less efficient". However, the key is that it only draws power when it's turned on, which isn't all that much. A mech fan has a parasitic load all the time, which is why an e-fan gives better mpg despite the conversion losses.

Almost all viscous fan -> electro conversions have made some mpg benefits.
I don`t know how much difference it does on a truck with a big engine, but people have been doing it on BMWs and Mercedes. The added benefit is also a faster warm up and, more so on a performance car, the willingness of the engine to rev more freely.

I have been wanting to do this for some time on my truck. The mechanical fan seems to stay engaged quite often, and seems to engage more in the winter than summer, which is odd. There is a noticeable drag on my engine when the fan kicks in. I saw an episode of Trucks! where the host was performing mileage modifications to his truck, and one of the things he did was replace the mech fan with an electric fan. He said he gained a little more than 1 mpg. Not bad.

1mpg on a truck that gets less than 20mpg is a huge gain. I was planning doing the swap also when it is warmer out takes forever for my truck to warm up with that huge grill opening.

if your fan is out of balance it will kill your waterpump bearings it would be nice to drop 10lbs off the waterpump nose.

Switching to an electric fan helps 2 ways. It improves FE, and it frees up power when you want.

My F150 has had no fan at all for about 3 or 4 years now. It rarely is sitting still in city traffic; as long as it's moving it's been fine, even when working hard. But then I haven't worked it real hard on a really hot summer day either.

i did this on my starlet 3 years ago, you will notice straight away an acceleration boost and that it warms up quicker, i suggest a hotter thermaostat while your at it, believe me it makes a difference in warm up times, as for wear on water pump, mines 3 years on and shows no signs of giving up so dont know where you heard that from.

when i did my conversion i gained 1.6kw on the dyno (thats a 3-5% gain for me
believe me it is worth it and it takes a load off the motor bearings as it has less mass to try and spin :)

if you want anymore answers feel free to ask no matter how stupid they sound :)