We (at our 30s) had a full swap (albeit not the first time) with our friends in a car.
Mid-sized car btw.
We had been out for dinner with a baby-sitter in both houses.
So, we stopped on our way back and changed seats. Me and his wife in frontseats, my wife with our friend in the back of the car.
The usual sequence of events: kissing, touching, fingering, oral for me. But the most complex was oral for her and finally, the big question: how to fuck with 4 in a mid-size car.
This did not fit. So, we all had our orgasms but through mutual masturbation.
Finally we all watched my wife finishing herself while I turned over and fingered her deeply.
After we had come home, we had sent the baby-sitters away, we again swapped houses and we stayed for some more hours before going to our respective "legal beds".
Good memories; now 30 years ago.