The definition of "bad" gas mileage can vary depending on the vehicle and driving conditions. In general, a vehicle that gets less than 20 miles per gallon (MPG) in the city and less than 25 MPG on the highway can be considered to have poor gas mileage. However, this can vary based on factors such as the vehicle's size, weight, and engine type, as well as driving habits and conditions. For example, larger vehicles such as trucks and SUVs are typically less fuel-efficient than smaller vehicles, and vehicles with high-performance engines may have lower gas mileage than those with more efficient engines. Similarly, driving habits such as aggressive acceleration and braking or excessive idling can also reduce gas mileage. Gas mileage can also vary based on driving conditions such as traffic congestion, road conditions, and weather. In general, vehicles will get better gas mileage on the highway than in the city due to the higher speeds and more consistent driving conditions. Ultimately, what is considered "bad" gas mileage will depend on the specific vehicle and driving conditions. If you are concerned about your vehicle's gas mileage, you can check the EPA fuel economy ratings for your vehicle or consult with a professional mechanic to determine if there are any underlying issues affecting your vehicle's fuel efficiency.