Can I go to Hong Kong via train or car from Guangzhou and fly out of Hong Kong airport?

Ho Chi Minh - Guangzhou - Hong Kong is well eligible for the transit, but the ticket leaving Guangzhou will be required for the processing.

You can leave Guangzhou to Hong Kong by train or car and then fly to US from HK.

Do you mean i can fly to guangzhou and have train tickets to hong kong?

There are no eligible trains for this purpose.

You have to fly to Guangzhou, but you can exit to Hong Kong by a through bus that goes all the way from Guangzhou or Shenzhen to Hong Kong. So, you have to book this bus ticket in advance and have the ticket or ticket confirmation with you in hard copy, ready to be seen by the Chinese authorities upon your landing in Guangzhou.