Charging your phone in your car has a minuscule impact on gas mileage. While technically every electronic component consumes fuel by drawing power from the alternator, which is powered by the engine, the energy required for a phone charger is very small—around 5-15 watts. This is significantly less than other car systems like air conditioning, which can draw thousands of watts and have a more noticeable effect on fuel consumption.

Reports suggest an average car charger uses approximately 0.03 miles per gallon. For a car getting 20 MPG, this would reduce efficiency to 19.97 MPG, a negligible difference. In contrast, air conditioning can reduce fuel economy by up to 10%.

Charging a phone while the car is running will not drain the car's battery. The alternator supplies the necessary power. However, charging with the engine off will draw directly from the 12V battery. While unlikely to flatten the battery, it's advisable to avoid this if the car hasn't been driven recently or if the battery is old. Overall, the fuel or battery consumption from charging a phone in your car is not a significant concern.