monte carlo

No way to increase MPG on this car, but a thermostat, plugged radiator, or low coolant/leaking could cause the overheating.

anything that will make the car more efficient will increase mpg id suggest a full tune up; air filter, oil and oil filter, spark plugs possibly new cap and rotor for your distributor, clean up the carb etc. oh and keeping your foot out of the gas will help

Adding a good set of headers and a k&n filter will help on the mpg and performance, Take everything out of the car that is not needed, even in the trunk. Make sure your tires are properly inflated , that info should be on the jam of the drivers door near the latch.

head gasket,thermostat or wrong fan

Maybe a new mass air flow sensor and your regular tune up could help with mpg, or changing ait intake filters besides that not much u can do. No comment on overheating