Around town fuel mileage versus steady state cruising fuel mileage should be a lot different, 6 or 7 MPH around town, depending on the driver versus 14.0 to 15.5 MPH at 55 to 70 MPH fuel mileage while cruising with minimum stop and go driving
As already suggested pull the plugs and look at them, the proper heat range helps make them easier to read and the ignition timing all contribute to your MPG.
I've work on one 383 two barrel motor like you have (1965 to 1971) and it didn't have very much true compression ration, maybe between 7.5 to 8.5 to 1 compression ratio
My message is you need a hot plug and a little more total ignition timing to make it run a good as you can, I would shoot for 12 to 18 degrees before top dead center at idle rpm up to 2000 RPM and then have 34 to 38 degrees total timing with a hot plug in the motor, a # 4 or 5 NGK or a RJ 12Y or RJ14Y Champion, Old Auto lite # 65 or 75.
What jetting does it have now and what vacuum rating power valve does it have, EI 4.5 or 8.5? That number is stamped onto one side of the power valves. up:
Holley are very sensitive to how they are jetted and which power valve they have and if the power valve gasket is installed and seated properly . That carb came with an # 8.5 primary power valve originally, #72 primary jet, secondary power valve was 10.5 and the secondary jet was an # 76
On the stock jetting it was design to be ran at sea level so for every thousand feet you are above sea level you should reduce the jet size by one Holley jet number smaller
Did that carb run okay before the rebuild? If so check the gaskets and power valves as well as the jet #, if not running good before the rebuild you will need to work on that carb to find out why it is doing what it is doing to cause your issues If you don't work on carbs find someone good that does, you will be amazed and happy once it is dial in correctly
As far as getting the true gas mileage you need to know exactly how many gallons of fuel used for each trip, I fill my gas tank up to make sure their completely full so I can see the fuel level in the filler necks at the same spot each time