Yes your vehicles does get better gas mileage when you use cruise control.
The reason you get better gas mileage when you use cruise control is that your vehicle is going at a steady speed and your throttle is at a constant speed and not moving back and forth.
When you're cruising at a constant speed you're saving gasoline but when you constantly press the gas pedal further and then release it and then press the gas pedal again the engine uses more gasoline.
The further you press the gas pedal the more gas is used.
So if you're traveling down the highway or interstate you can save gasoline by using the cruise control.
I use the cruise control on my vehicle when on long road trips and it does save gas and allows me to move my leg around to prevent my leg from cramping up.