With gas prices rising, it is no wonder drivers are searching for new ways to save money on gas. However, some people have raised questions about how bad batteries affect gas mileage, asking, "Will a bad battery cause bad gas mileage?" The answer is yes. A defective battery can result in poor gas mileage. Unless a battery is properly charged, fuel injectors in a vehicle are unlikely to push fuel into the cylinders normally. Due to this, drivers are more likely to lose fuel mileage if the alternator runs continuously. So, if you notice your vehicle running a bit rough or your car engine is jerking, check it out to avoid further impairment.