Yes, it would certainly help if they had someone that could do simple math :P

The only thing I can figure is that John sent Dean on a hunt that required a vehicle that could handle rough terrain, so Dean would be better off in a truck rather than a car. Or John could have simply said, "hey, I miss driving that car, I'm going to take it on a road-trip for old-times sake, and since it was mine first, you can't argue!" And Dean would have been like "Yes, sir :("

I know someone who belongs to a family with three guys in it, and one time I visited him when they were all home one summer. One day we were out and my friend kept fidgeting all day until finally towards the end of the day, he said "you know, I don't think these are my underwear."

So, yeah...clothes sharing. :P