You don’t need to make the trip to Cambodia. You will get 60 days visa exemption on arrival and can extend it by 30 days at an immigration office.
Also be aware that the airline can ask to see your exit ticket out of Thailand and it has to be within the first 60 days. Immigration, of course, have the same right.
If the car you are planning to drive is a rental car, you may find there are restrictions about taking it out of the country.
I think this is quite a common mistake people make. Immigration are probably used to it by now. Can just pay a 500 baht fine but you may get a stamp in your passport.