the first one
It depends on how many Johns and how many cars. "Someone went into the parking lot of the cathouse and stole all the Johns' cars' wheels." "John has fifteen cars; John's cars' wheels are worth more than the whole of my car." "Our car has been purchased by Elton John and Olivia Newton John, and believe me, the Johns' car's wheels stay on the ground." "My friend John has just bought a car but I think John's car's wheels need replacing."
The sales tax on a car is 6% for anyone that lives in the state of Florida.
No, you're in the car not on it.
James's car
The correct GAUGE wire is 28 for a car stereo.
Yes, perfectly correct.
Yes. "The car that just passed was theirs" is grammatically correct.
Val Johns's birth name is Valjean Johns.
Daniel Johns's birth name is Daniel Paul Johns.
Margo Johns's birth name is Jessie Margaret Johns.
Depending on what "it" is, "took it off the car" can be correct English usage.
The grammatically correct way would be: To whom did you sell your old car?