Cars vary a lot in their fuel consumptions, from about 18 miles per litre (about 80 miles per gallon) for a small diesel saloon to five times less for a large 4x4 vehicle. The average is about 8.6 miles per litre for all cars (or 39 miles per gallon).
So you can reduce your CO2 emissions and save money in fuel costs by buying a fuel-efficient car. The average petrol car in Britain achieves 28 mpg in town, 47 mpg in the country, and 39 mpg combined. Many small cars now available can achieve 80 mpg and large family cars can achieve 65 mpg for combined driving. Electric cars, or course, are even better (though unless you buy renewable energy, even though they do not emit pollutants through their tailpipe, they are still responsible for some emissions from the electricity generation).
Thinking about carbon dioxide emissions, the average car produces around 200 gm CO2/km. It is now possible to buy new small cars producing less than 120gm/km. You can buy a new family car or a top-end quality car producing less than 150 gm/km. A car rated as less than 100gm/km costs nothing in vehicle excise duty; less than 120 gm/km a car has no vehicle excise duty in the first year and £20 or £30 a year thereafter.
Manufacturer figures about fuel economy are unrealistic. Compare them with the independent figures given by Nextgreencar http://www.nextgreencar.com/ or Fuel-economy http://fuel-economy.co.uk/stats.shtml.
A word of warning… the emissions from manufacturing a car can be very large (about 6 tonnes for a small car and a whopping 35 tonnes for a large 4x4 car). So scrapping a car before its useful life is over for a new and more fuel-efficient one will actually increase your CO2 emissions for a period until your accumulated emissions savings from better fuel consumption exceed the emissions from manufacturing the new one.