I have an Edlebrock 1412 (800cfm) carb on my 408 stroker.
When I put my car on the road back in 2001, I naturally got the WOT mixture dialled in properly on a rolling road. Two jet sizes up on the secondaries and it was fine. I never really gave much thought to the cruise mixture, except that a year or so later I richened it in an attempt to cure a slight stumble. It didn't improve but my attention wandered onto other things at the time, and I never went back to it - after all it was running pretty fine. I fogot all about it.
I started to read the manual properly(!) a month or so ago. One of the things it says is to keep leaning out the cruise mixture until you get a problem, then go back a stage. I never did that before, so I decided to do it now. With these carbs, you can lean out the cruise mixture without affecting the power mode. A little concentration, and the graphical representation of this in the manual made perfect sense.
A couple of weekends later, I am 16% leaner in the cruise (with the edlebrock it is dead easy to change rods and jets) and she is running better than ever. What this will translate to in terms of mpg I don't know yet, but it will probably be very +ve. Oh - and the stumble I referred to earlier is gone (was getting far too rich with the accelerator pump shot - which I have also reduced now).
I really don't know why I did not do this earlier, wonder how much extra fuel I have paid for in the meantime?
So.............RTFM, do what it says, and save some money.
At this rate I will qualify to join the Green party (shudder). Cruise mixture can be very much leaner than you think, without any issues.