Can I fly to Guangzhou from HK but come back to HK by car. I will have a HK flight.

No, you cannot do that. The first leg is fine, but the second one is not acceptable because of the following two reasons:
- You cannot return to the same country / territory, but to a third one
- You will have to have with you, already in Hong Kong, a ticket for the public transport to a third country or territory. Cars and trains are not acceptable in Guangdong province.

The solution for you is to return to Macau that is so close to Hong Kong by ferry from Shekou port in Shenzhen directly to Macau. This route is maintained by TurboJet ferry company. If you do that, you will be eligible for 144 hour visa free transit valid for entire Guangdong province.

If you decide to accept this option, you will have to buy the ticket for the ferry I mentioned, before your departure from Hong Kong, print it out and have it with you. Also, you will have to use exactly that ferry you have bought your ticket for before your departure from Hong Kong.

After your arrival in Macau, you are free to return to Hong Kong, of course.