[Firebirdjones]

The Quadrajet will return better mpg for a few reasons. The front primaries are smaller, a very efficient booster design atomizes the fuel, and the metering rods offer very fine fuel metering.

Without a wideband AFR to do your tuning with however,,,you are basically shooting in the dark. With the right lean part throttle adjustments made using a wideband, you can get great mileage out of most any carb,,,some are more involved to deal with, but the end results are similar.

EFI shoots for a target 14.7 AFR all the time unless you are in PE mode (power enrichment).

You can shoot for this same AFR using your wideband and in some cases even a little leaner doesn't hurt anything on light throttle cruise, light load conditions. Some tuners have gone as far as 15.0-15.3 AFR for light cruise applications and pick up some MPG. I've flurted with 15.0 on some of mine, and it's been fine for years. On my 502 pickup however I prefer to keep that one slightly on the rich side (14.2-14.4) at part throttle because I use it for towing heavy loads. At low vacuum conditions (half throttle or more) when towing up hill or pulling away from a stop, the power valve opens and quickly richens it to the low 13 AFR range, and full throttle is at 12.7 AFR, like I said, slightly rich but I feel it's better protected from detonation this way while towing.

The problem now though is ethanol in the gas. The more ethanol in the fuel the richer the fuel mixture has to be to make the same power. Even 10% will change the AFR .3-.4 tenths so there is a fine line there,,,and then it gets more involved depending on where you fuel up the car. Not every station carries the same amount of ethanol. I've seen them carry as much as 17% using a little ethanol test kit. So gas mileage, (and you tune) will change from one fillup to the next.

As far as a carb working better than an EFI is up for debate. The advantage EFI has here that I find tuning my LS engines, is the very fine detail fuel adjustments that I can make in the fuel table cell blocks which creates an entire graph or fuel curve. I can make it do what ever I wish and get some pretty darn good fuel mileage out of it with a key stroke. The adjustments are infinate. On a carb there can be certain areas that you'll have to fudge and live with where the AFR dips or jumps around,,,unless you run a very expensive carb that has adjustable air bleeds, adjustable power valve circuits etc....and it gets pretty involved.

However it has been shown on the dyno with the same engine that a carb still makes more HP and torque by a very slight margin.

They both have good and bad points.