-electric fan vs. engine driven clutch fan
-Cold Air Intake
-Programmer/tuner (which now I'm hearing is BS about the gains in MPG)
-gears(in a way for bottom end, torque)
-throttle body spacer(also questionable)
The electric fan is a good idea. You may gain 1 or so mpg, maybe even a couple. I wouldn't be overly optimistic about it though, the gain will most likely be small, but still worth it for all the benefits.
CAI will maybe add a little, most people I think exaggerated the gains though, they are minimal in terms of mpg.
A programmer will give you the most by far. They are definitely worth it and are not a gimmick. I picked up at least 1 mpg city and hwy with the 93 performance tune on my truck. I am told I can get the same mpg with the 87 tune (cheaper cost/mile) but I like the performance. Depending on many factors some people claim up to 4mpg with a tuner, that's pretty good. I bet the programmers are the most realistic gains also. also they are not really any more expensive than any other mod.
Gears can both help and hurt, so it depends...
TB spacer does nothing.
If you did several of these mods I'd say you would see a realistic 2mpg gain...
It's very unrealistic to do 5 or 6 of these mods and expect the mpg gains to stack up to 50% of what original mpg was...