The lack of leg-room and the fact that he had ridiculously long limbs was the reason that Sam hated sitting in the backseat of the Impala. Especially when there was someone else sharing the seat with him.
If he was alone in the back, it wouldn't be too bad. He'd lounge with his legs up, sprawling himself completely over the bench-seat. Most of the time he'd doze off, mostly because the reason he was in the back seat was because he wanted to catch some sleep while his brother drove.
Those times lessened when Dean and Castiel began dating.
His older brother would insist that his angel should sit in the front with him, forcing Sam into the rear.
It was fun, at first, allowing Sam to relax, without the need to catch up on sleep. He would read books, listen to his music on his iPod, and present that his brother and his boyfriend weren't being mushy with each other in the front seats.
After a while, it got kind of old, especially when Charlie had joined them on a hunt. Still banished to the back, Sam folded his legs up and wedged them into the foot-well, his legs beginning to cramp after only a few minutes. He was grateful that the hunt was only a short drive away, and gave himself a few minutes to stretch out his cramp, before they began to investigate.
Sam relished in the times when Cas didn't join them on hunts, and therefore the Impala, when he got to sit in the front again, and stretch his legs freely.
Dean laughed at him, and told him his moose limbs were too big for a human-sized car.
—
They spotted and picked up Cas on the way back to the Bunker, after a hunt, but because Sam was already in the front seat, asleep against the window, the angel climbed into the back. When the younger Winchester woke, and saw the sad look in the blue eyes he could see through the rear-view mirror, he knew he wouldn't be able to sit in the front with Dean if Cas was ever in the car again. So, the next time all three of them went out on a hunt, he didn't even try to defend his right to sit shot-gun, instead climbed automatically into the back.
He finished many books during his time in the back, and he had never been so relaxed, after a few hours lounging there. It was like his own personal chill-room.
Dean didn't even complain about his feet being on the leather anymore, which Sam counted as an extra bonus.