I enjoyed visiting Hendrick Motorsports which is close to Charlotte Motorspeedway where they have the Nascar Coca Cola 600. Admission is free as I visited 3 buildings here. The first was a museum of race car winners which mainly included race cars won by Dale Earnhardt Jr, Jeff Gordon and Jimmy Johnson. There were other older race cars here too by other race car drivers. In the museum is a gift shop where you can buy all kinds of Nascar memorabilia such as T shirts.
The second building I visited was the 48 / 88 building. They did not have a viewing window here where you can see mechanics working on race cars.
The 3rd building I visited was the 9 / 24 building where they did have a viewing window where you can see mechanics working on race cars.
My Experience: You are pretty much on your own if you are just Nascar fan. You just come in and nobody talks to you are guides you. Now, you can walk up to the counter and ask questions like did and they will help you.
Final Thoughts. You can not take a tour of the mechanics shop unless you are a sponsor or work for a sponsor. I did see a lucky few getting personal tours of the shops. Overall, I liked looking at the race cars but wanted some interaction. It would have been nice to have a tour. I would have paid a reasonable amount to go inside the race shop and watch mechanics in action. They could rope it off so we fans would not interrupt mechanics doing they work.