Daughter needs Gas mileage from 273 commando

The air valve secondary carb suggestion, with the D4B and an overdrive would be ideal.

I bought the new Street Demon, because it's based off of a ThermoQuad, but has a better idle circuit and a redirecting air bleed valve that works in conjunction with it's sandwiched gaskets/ baseplate to redirect air at the primaries, to the center of the intake and further help with atomization.

I was able to tune within increments of 30rpm on that carb and I've never heared an engine idle so smoothly. It's a 625cfm carb, but the primaries are smaller than my Carter BBD 2 barrel.

They sell them with metal and composite bodies. The composite lowers the fuel temp to help with charge and the exhaust crossover in the D4B will help atomization.

Multiple spark is also very helpful, along with tuning the carb on an o2 sensor and reader. That would help any carb tune to proper AFR without doing it blind.

Ultimately, an overdrive would help you more than anything, along with driving habit, which can be measured and corrected by monitoring a vacuum/ economy gauge.

The idea behind the closed chamber, tight ports and small primary is to dump as much torque into the engine as you can at low RPM, so it doesn't need much assistance from throttle position to get the car moving. The less the throttle plate moves/ more the car moves, the better fuel economy you will see.