Creations, differently from others, we have the same night length, with or without a nap, which is approx 11 h. Due to daycare and working schedule, I can't do EBT, so we have always worked with fixed bt, and he is self-regulating, in the sense that he's offered a nap every day (at daycare kids of 3 and 4 are required to nap) and he "decides" to take it or not. Now, he's having an average of 2 nn days every week, and these days bt is half an hour before, but he usually wakes half an hour before in the morning, so he doesn't tackle. I think that these two nn days are needed to keep the night long enough having a 1 h nap the other days. I decided I am fine with this, until he's happy (and he usually is) and his nights are longer than 10.5 h. His sleep needs dropped to 11.5 h per day in average (11 h night+1 h nap or 11 h night without nap), and it seems to suit him. He's older than your ds though.
About this:
On 3 of our no nap days he's been in the car at nap time (planned to let him fall to sleep there then transfer to cot for the rest of the nap) and not been the slightest bit lulled to sleep. Even yesterday on day 2 of no nap I had him in the car at 2.30 (nap is set at 2pm so it was already late and he was showing no tired signs at all) for a good 20 min drive and he didn't even nod.
From about 2 yo it was almost impossible for us to let DS sleep in the car, even when he clearly needed it. He started to be able to stay awake if he wanted to, and in the car he had too many stimulation, looking outside of the window, to be willing to sleep. Now, that he's older, he only falls asleep if he wants to. So we have to explain him that we need him to go to sleep in the car, so after we have time to play/going somewhere, otherwise we will need to put him down for a nap and there won't be time for enjoy... if he's convinced, he closes his eyes and falls asleep. Otherwise, no way.
What I mean is that if he's used to fall asleep in the cot in the dark and he's a IS, he can become able to resist to sleep in the car.
