how many cars are on the road?

hey everyone-

i'm writing an environmental impact research paper and i need to find a reputable webpage that lists the number of cars on the road in the united states.. it'd be good if there are a few years listed so i can compare.. apparantly the nation's avg. fuel economy has gone up by about 8mpg over the past 20 years. i want to see if the number of cars on the road has effectively cancelled that out or not..

anyone with some leads?

-adam