system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5:Why do I get significantly less gas mileage using cruise control compared to not using it?
gpt
Cruise control can be inefficient if you have an extremely hilly drive.  To maintain a constant speed, it will accelerate on the uphills and brake on the downhills.  This can kill your mileage compared to coasting and allowing your speed to vary.

If your drive is rather flat, though, then that shouldn't be happening.