Several things can cause the smell of gas fumes inside a car, including a leak in the gas tank. A missing gas cap can also cause the smell.
The first step in identifying a gas odor inside a vehicle is to confirm that the gas cap is in place and tight. If the cap is in place, the underside of the vehicle should be checked to determine whether the gas tank has a leak. Another way to check for a leak is to look for spots on the pavement or driveway where the vehicle is typically parked. If a gas tank is leaking, it should be replaced or repaired as quickly as possible.