fan question

[67ChevyC10Stepside]

ok...i have a 67 short step with a 400 SB in it...wont be doing any towing probably wont be driven on any real long trips or anything but when it is driven will be driven hard. my buddy has a 87 shortbed 305 4wd truck that tows a i think 30 foot horse trailer....and they also have a 85 GMC truck with 350 4wd that tows the same horse trailer...both of those are driven pretty hard....now to the point at hand...the 87 recently sliced a rad house on the fan so when we replaced it we had a electric fan out of some little car laying there so we decided to put it on...with the mechanical fan it got 8 MPG just driving around....with this electric it got 9.9 towing the trailer...so we are gonna buy electrics for all our trucks...not only for that reason...the mech fan gets im my way looks ugly etc etc as well. but the question is...what kind of CFM can a 5 blade clutch fan put out? and what CFM do i need for those trucks mentioned above...im looking at a 2400 and some 2750s...the 2750 is roughly 15 bucks more each than the 2400....sorry for this being so long.