Actually as far as tv ratings NASCAR is second only to the NFL. Plus it's the largest spectator sport in the US.
I had to look it up. And this is what I found. NASCAR is ranked 9th #9 Motor Sports Motorsports (auto-car and motorcycle racing) come next in the list. Motorsports became famous in America in a very small span of time after getting introduced as an official sport. NASCAR is the biggest organisation in USA, which organises motor sports races. As of 2011, it was the second most watched sports on television in America (after Football). There is a very popular museum in the USA named as Motorsports Hall of Fame.