2 Answers

Wiki-pedia is not up to date on the numbers. Last recorded number was 2009. 

According to the US Bureau of Transportation Statistics for 2009 there are 254,212,610 registered passenger vehicles.

Number of Cars in America

No one knows how many cars there are in America. Many are not registered and are stored, in salvage yards, and some just sitting idle. But, there are approximately 250 million registered vehicles on the road today. That figure includes all types of vehicles.
Approximately 16 million new vehicles are sold annually.
Here is an estimate using simple algebra
C: number of cars
P: number of people: P=305,541,491 as of 3 Jan 2009
C < P: the number of cars is probably less than the population of the United States, since only the rich have more than one car per family, and many families have no cars.
Cn = P where n is > 1 by the above reasoning
C = P/n (d'oh)
Let N = 5
The number of cars C is therefore 305,541,491/5=61,108,298
We figured this out before looking at the answer of "sixty-two million registered cars".
Pretty slick!

The answer for 2007 is 136 million cars, 110 million trucks, about 1 million buses for a total of 247 million registered 

source: wiki answers