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My question is, what are the opinions of others on how much fuel might be saved using an electric fan vs. mechanical, and how did you come up with your figures?
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I did not even realize that there were any vehicles made in the last 15 years that did not have an electric fan. However, here is a link that says 16 to 41 hp used for big trucks.
A comparison of viscous drives and on/off fan drives for diesel engine cooling | Diesel Progress North American Edition | Find Articles at BNET
I think what you did is worth while. If you had the opportunity, you could measure manifold vacuum at a particular speed on a particular stretch of road with the old vs new cooling fan. Then, with the electric fan in place, go back to that road, and speed up till your manifold vacuum is the same as it was with the non-electric fan.
Now you know (more or less, not exactly) how much faster you can go and use the same amount of gas. If you do a roll down test of the truck you can calculate .cd, and from that calculate the power it takes to move it at different speeds. From that you get the power the old fan took, and with a guess at engine efficiency, you can back out the value of MPG.