I picked up my Niro in the dead of winter and my first fill was 42 mpg, but included fast interstate trips at 75 mph. Until May, never got above 50 mpg. Now I'm getting 58 mpg in hot weather not exceeding 65 mpg. I also have the base narrow tires.
If you have the heavier model with the wide tires, getting low forties with high speed commutes is expected. Thirties is really bad! Are you leaving it in Sport mode? Still, I bet it is better than your last car presumably driven similarly.
2018 Kia LX HEV Metal Stream with Advanced Tech