The smell of gas fumes inside a car can be from exposure to gas vapors and spills, gas leaks, irregular fuel pressure. Or in the case of an old model car, it can be from fuel boiling in the carburettor float bowl.
Fuel leaks can occur on fuel feed lines, fuel injection lines and fuel tank vents. These can be detected if gas puddles under the car start to appear. Irregular fuel pressure makes the fuel mixture either too rich or too lean. If the fuel mixture is too rich, the car burns excess fuel which produces a higher concentration of gas fumes in the exhaust and increases the smell of unburned gas.