How Much Does Driving Fuel Efficent Cars Cut Co2 Emissions Over Driving Gasoline Powered Cars?

Depends on the car!
In Europe, CO2 emissions per km are advertised with the vehicle, in the case of petrol vehicles. The new Prius weighs in at 89g, and the most efficient diesel cars are the Smart fortwo at 88g, and the ‘eco’/'bluemotion’ versions of the Ford Fiesta, VW Polo etc, which are all just under 100g.
A ‘normal’ car of about Prius size, say a Ford Focus, gives somewhere between 120g and 150g/km, depending on the model.
Electric cars depend on how they’re powered. The Chevrolet Volt claims it’ll use an 8.8kWh battery to drive 64km. In the UK, each kWh currently produces about 430g, so the Chevy Volt in electric mode produces about 60g.
If powered by nuclear or renewable energy (eg France is about 80% nuclear iirc), then emissions would be about 3g.
If powered by coal, it would be about 124g.

First of all you don’t say at what level you are writing this paper. Is it your 7th grade homework, a doctoral thesis or research for your latest book?
So I will strike somewhere in the middle. Your terms “green car” and “average amount saved” needs to be defined a little more carefully.
What is a green car and what will you consider when adding up the “average amount saved? Is it one that operates with reduced exhaust emissions or no exhaust at all (like an electric.) From this simple point of view you only need to know that when a gallon of gasoline burns it produces about 20lbs of CO2. This is more than the gallon of gasoline weighs because most of the weight of the CO2 is oxygen from the air that has combined with the carbon in the gasoline.1 You can calculate the amount carbon dioxide produced by simply calculating the amount of gasoline that is being used. A vehicle that gets double the mileage will produce half the CO2. The total amount will be equal to 20# times the number of gallons used. Electric vehicles produce no CO2 in operation.
Two basic principals will always apply. If a vehicle uses less fuel it will produce less CO2 than a similar vehicle that uses more. And the one that is more efficient will use less fuel.2
Are you also considering how the energy for the vehicle was ultimately produced and transmitted? This is where it gets more complex as not all fuels are equal. There are three considerations:
-how much energy does the fuel contain
-how much carbon will using it produce
-what is the efficiency of the production, transmission and use.
Gasoline contains about 114,000 btu / gallon but the vehicle we use it in is usually less than 15% efficient. 85% is a carbon cost that we are not using. Oil refining and transportation doubles the use of petrochemicals and produces a 1/3 waste product. Biodiesel produces three times the energy required for production.4
The electric vehicle is around 90% efficient. Solar Cells will not produce a lot of energy but will also produce no carbon in their use. In the US a little less more than 45% of the electricity is from coal and there are losses in transmitting power that reduce efficiency a slight degree. In other countries more than 85% of the power production produces no carbon.5
Or are you also considering the manufacturing of the vehicles? A bicycle is going to take less CO2 to manufacture and transport than a Hummer simply because the process is less complex and energy intensive. This is going to be true even if the hummer is converted to run on electricity. This is the most difficult carbon dioxide production to determine with any accuracy.
A hybrid vehicle will fall in between an electric vehicle and gasoline engine vehicle in fuel economy, efficiency and carbon production. Not all hybrids are equal. As they become more popular some will be produced that are not particularly efficient and may have an increased carbon footprint due to increased production costs.

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