If it looks dirty, the small cost of a new one is quickly repaid. Change your oil. Don't leave the oil in your engine after the specified change time. Used for too long, oil thickens up and begins to waste petrol. If so, it wastes fuel.
Check brakes, wheels, steering.
Check all three. Wheels out of alignment, steering incorrectly toed in or out wears tyres and wastes petrol. Dragging brakes eat fuel. Be sure that your gearbox and differential are filled to the correct level with the right lubricants. But don't overfill - it consumes extra power.