Recirculated air is generally cooler than fresh air, the greater the temperature of the air passing through the evaporator core the greater the load on the system. A high load on the system means higher pressures for the compressor and a greater load on the engine.
In very hot weather I usually switch to recirculate when stopped at a traffic light, the outside air in a group of idling cars is 125 F plus. I switch to fresh air when moving, continuous use of recirculate will pollute the system and lead to foul odors.
By simply switching from fresh air to recirculate you will not be able to feel the difference in the load on the engine but when driving a car with a small displacement engine there is a noticeable difference in load between having the A/C on or off in 110 F weather.
Difference in fuel economy? You won’t notice with your old car but some late model vehicles change the A/C operation when “ECO” mode is selected so manufactures feel that there is a way of saving fuel with a reduced load on the A/C system.