I have 1973 280 with a Weber carb and was getting super low fuel economy. It started at 7mpg then I managed to tune the carb so I was getting 11, then after park break adjustment I'm at whooping 14!
Here are some factors that influence your fuel consumption (in no particular order):
-- proper distributor timing
All adjustments out of specs can cost you fuel economy, even something as basic as advancing or retarding distributor. There is a curve where if you go from retard to advance you gain fuel economy then starts loosing it when you advance too much. Bottom line is stay within factory specs and make sure your mech advance is working properly and to specs.
-- old sparkplug wires or damaged distributor cap
Cracks in the insulation can cause the electrical current jump to the engine housing rather then be delivered to the sparkplug. So your engine sucks in the fuel to the cylinder but then spits it all out unburned through the exhaust. You not only loose the fuel but also power.
-- wrongly gaped or old sparkplugs or old / damaged coil or capacitor
Old coil can't deliver spark hot enough to jump the sparkplug's electrodes causing intermittent misfire and unburnt mixture going out of your exhaust. Old or carbon fouled sparkplugs can cause misfire too.
-- (carb) wrong fuel mixture
Wrong fuel mixture - controlled through jets, idle screws and float level is bad (too lean or too rich). It causes misfire pushing gas/air mixture unburned (or half burned) out of the exhaust pipe (robbing you of fuel and power).
-- (carb) float level in the carb adjusted incorectly
Wrong float level can cause excessive fuel consumption. Too low = lean, too high = rich.
-- (carb) leaking fuel needle valve
Leaking valve can cause fuel bowl to fill excessively (despite proper float level) causing the carb to run rich.
-- (carb) wrong fuel pressure
Too high pressure can force the fuel past the needle valve causing the carb to run rich as the fuel bowl overfills.
-- (carb) wrong fuel bowl venting method
If the carb is venting to outside while driving - your fuel economy will suffer. You want the carb venting internally when driving then to the outside when the engine is off (to prevent vapor lock).
-- stuck EGR valve
EGR valve stuck in open position causes constant lean burn thus you need to open up throttle to deliver power equivalent power.
-- dragging breaks or misaligned (dragging) parking break
This should be obvious too but took me ages to properly align.
-- tires that are under inflated
This should be obvious that tires that are too "soft" require more force to roll the car. Again keep those within specs (not too inflated either as tires warm up).
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