Under inflated tyres create more resistance when your car is moving. That means your engine has to work harder, so more fuel is used and more CO2 emissions are produced. Simply check and adjust your tyre pressures regularly and also before long journeys. This will also help to increase the life of your tyres.
Under inflated tyres increase CO2 but over inflated tyres can be unsafe, so check your car manual for the correct tyre pressure. Remember, a car with a heavier load may need different air pressure in the tyres.