A few days ago I posted 2008 Most Fuel Efficient Cars. And here is another way of looking at the numbers:

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Now lets look at it this way; on average we may drive 10,000 miles per year (50% City & 50% Highway), and let's just average out gas at $4/gallon (Current in my city it is $4.69). So the yearly numbers become:
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Toyota Prius Hybrid / City: 48 mpg / Highway: 45 mpg
Yearly Fuel Cost: $860

Typical Small Car / City: 30 mpg / Highway: 35 mpg
Yearly Fuel Cost: $1,238

Typical Large Car / City: 20 mpg / Highway: 30 mpg
Yearly Fuel Cost: $1,666

Typical Mini-Van / City: 18 mpg / Highway: 25 mpg
Yearly Fuel Cost: $1,911

Typical Large SUV / City: 12 mpg / Highway: 20 mpg
Yearly Fuel Cost: $2,666

With these simple parameters; if you switch from a large SUV to a Toyota Prius, and even if you drive a modest 10,000 miles per year, you'll save about $1,800 per year.