Yes using cruise control on your vehicle when you're driving down the highway or interstate does help you save gasoline.
Your gas mileage should improve by a lot when you use cruise control which has been my experience especially when traveling down the interstate on long road trips.
The cruise control helps you keep your foot off the gas pedal and maintains a constant speed and throttle position.
When you have your foot on the throttle you usually move the gas pedal back and forth a little even without realizing it.
When you do that then you use more gasoline.
My gas mileage improves by at least 3 miles per gallon when on road trips and using cruise control.
It's nice to use cruise control to help keep your foot off the gas pedal which prevents leg cramps because I can move my legs around.