dunno, I haven't been at all scientific about this. I've just done everything at once. aired up my tires past mfg spec, did the aero mods, and started frequently shifting to neutral with a lot of P&G. I've also been keeping my reps below ~1750 (which with 4.10 gearing means a top speed of 55). however, I filled up my tank when I first started making changes, did not top off, and at 150miles am just now bouncing around 1/4 tank down. normally I'd only be at 100miles by this point, so there's definitely been an overall significant improvement. maybe whenever I put in an MPG gauge I'll do some A-B-A and isolate specific mods.
reminds me, in your silverado, have you noticed the fuel gauge is unreliable? especially near empty, it fluctuates in a way that doesn't seem to correlate with fuel splash, although I've seen that too. what I've also found now is in neutral is will read a little different than while in drive. I know there's a common problem with the sending unit with these trucks that can cause a false empty, and that others have seen neutral involved, but have yet to come across an explanation for these particular variations.
