The phrase "worse mileage" means that a vehicle is not able to travel as far on a specific amount of fuel compared to another vehicle. In simpler terms, it means the vehicle doesn't get as good gas mileage. Full definition
The flex - fuel car gets worse mileage; the government credits it with improved mileage. (extremetech.com)
At least it's on regular and not premium gasoline; the need for premium is the equivalent of getting 18 percent worse mileage at today's fuel prices. (extremetech.com)
Some reviewers complain about how bad the mileage can be, but its all in your driving habits / style. (cars.com)