2B vs. 4B

If you're like most of us, your mileage will go down a little due to the fun pedal's increased use...
But... you may get an increase in mileage. Your 2bbl is likely a 390cfm. If you go to a 4bbl (say up to a 600cfm carb), your primaries will only be up to 300cfm, which is somewhat smaller. My Autolite 4100 is a 480cfm, and I can get anywhere from 12-18mpg in town (life's too short to drive "like a grandmother", and between 15 and close to 30mpg on the highway. I converted to a five speed (T5Z with a 3.00 rear and a carbed 5.0L :o ) so your mileage will vary.
Daniel

I'm getting 12-14 in the city and 15-17 on the road with a mild cam, 500 cfm 4 bl Edelbrock Performer, c4 auto, duals and close to a 3.0 rear ratio. Not sure on the rear end. It is great for fun but is definitly not a race car.

BornInAFord nailed it. The key to better mileage with a 4 bbl is how well you can control your right foot. My 331-sixpack Ranger has gotten 16 mpg with 3 Holley 2 bbls on it, as along as I drive it conservatively ( the speed limit everywhere and keeping my foot out of the two secondary carbs) If I drive it the way I like to, it'll knock down 12 mpg all day.

You are right about gas consumption, but if you upgrade other parts of your engine your gas mileage will go up too. I used to get 15 mpg (highway) with stock engine. Now im almost at 20 because of 8mm spark plug wires/new spark plugs, pertronix ignitor and 12v coil, and new air filters. It would also help if you upgraded your cooling system on your engine.