Disclaimer: I don't own Supernatural.
Just because he wasn't his Sam, didn't mean he wasn't named Sam, so reluctantly and only in his own head did he start calling the man Sam. Sam at the moment was driving, that's all they did every day was drive from one place to another. It had been 4 weeks of this, 2 more weeks of this and Sam would find out the truth. What would Sam do when he found out they weren't brothers? He didn't ask because honestly at the moment he didn't really want to know. They sometimes stopped for fast food and Dean would endure the short full body paralyzing if it meant hamburgers instead of granola bars.
Today though, today was a granola bar day. The sky was dark and rain was splattering onto the windshield. Every time the brakes squealed or a car honked he would jump. He really didn't want anyone else driving his car but him, especially in dangerous conditions like these he really didn't anyone driving his baby. Of course Sam merely laughed every time he made any comment about it. He saw why a few minutes later though. A car was swerving directly into his baby. His heart plummeted into his stomach, but he couldn't seem to look away. Only to have Sam waved his hand and have the car swerve back into the other lane. Dean was stunned and turned to look at Sam who smiled at him smugly, then put his eyes back on the road.