One of the most important things you can do to reduce your carbon emissions from travel is to reduce the number of times you use your car.
This is obviously easier for shorter journeys, but also more relevant - the heaviest car emissions occur during the first two miles that a car is driven, while the engine is still cold.
Before setting off on a journey, think if you could walk, cycle, or take the bus or train. Alternatively, could you combine errands, and save a journey later? Could you plan out the shortest route possible when combining errands?