GM makes this handy dandy fuel economy calculator that makes it really easy to gloat about how much money you can save riding a scooter. Check it out: http://gmdynamic.com/gmfuelcalc/
Here's a sample SUV:
ACURA, MDX 4WD, 6 cyl, 3.7 liter, Auto(S5) , 4 wheel drive,
Fuel Type: Premium
Vehicle Fuel Economy (MPG)
City = 17 mpg
Highway = 22 mpg
Combined = 19 mpg
Annual Fuel Costs Based on:
Premium fuel $3.21
15,000 miles
55% city-45% highway
Annual Fuel Cost = $2534
Annual CO2 Emissions (in metric tons**) = 7.0
(Based on...15,000 miles per year or 789 gallons of fuel purchased)
So I figured how many gallons it would take to travel 15,000 miles at 70 mpg on my Vespa and I come up with 214 gallons. Gas, at the same price as above, for those 15,000 miles would cost $687.85. A savings of $1846 per year just for fuel! And that's here in the states where petrol prices are relatively cheap.
So, by extension, if you drive a scooter the same amount of miles as you would have driven in a car, the scooter pays for itself in about 3 years, just for the price of fuel alone!