We can usually transfer our 21month old from car to cot, but very rarely from car to pram.

I get him ready in the car- usually before i drive, like take off his shoes and pants (if he has jeans or uncomfy ones on). I usually leave his blinds and curtains closed when i leave to go out anyway, but if there not i'll quickly get those done. Then as i'm getting him out of the car i'm constantly shhh'ing and try to keep his head out of sunlight. Then lay him in bed, turn his musical seahorse on- so he knows where he is and sneak out.

But i do find it can depend on how tired he is and what sleep stage he is at. Same as Shanti said- anything over 15-20mins it usually doesn't work (unless its night time or he is really, really tired).

Just an idea if its possible if your having trouble with their sleeping during the day- have you tried laying down with him. I've done this when all else has failed and it usually works. I think he enjoys the hair stroking and tummy tickle- puts him to sleep within minutes. Then if you've got things to do wait for the deep sleep signs and sneak out.