Fuel economy is totally dependent on mixture quality. Good quality A/F mix and good economy.

The AF ratio (except in EFI systems) is controlled by the amount of vacuumn caused by the velocity of air through the mixing device (Gas Mixer or carb). Ideally the AF ratio is constant. In reality it varies with air velocity (throttle opening) through the mixing device and engine speed. It is not only air fuel ratio that has an effect on economy but the quality of the air/fuel mixture (fuel particle size and the degree of uniformity through the mixture). Finer particles, (evenly distributed so each molecule has easy acces to oxygen for combustion) maximises the amount of heat per burn cycle.

At high engine vacuumn you have high air velocity through the mixing device. This gives better fuel quality through better mixing (emulsification).

Assuming an ideal spark condition, by finding the throttle/engine speed combination where the AF ratio is at the ideal mixture with best emulsification, you will achieve the best power/economy compromise.

Suck through turbo's on LPG get very good mixture quality due to the gas passing through the compressor turbine and so can give very good results even on low boost levels.

Blow through systems mixture quality is no better than N/A carbs or mixers but is able to force more mixture into the cylinder.

The vacuumn gauge will give you an idea of velocity through the mixer.