A month ago I was giving a ride to a Lowell bar. At a stoplight me and the PAX watched two groups of neanderthals on the sidewalk square off and start fighting , Lowell PD arrived moments later and pepper sprayed 2 of them as they scattered.
From 15+ feet away we smelled and tasted the spray. I quickly closed my window that was only open a couple inches. I'm not very experienced with the stuff, but if it spreads in the open air that quickly, I think if you sprayed someone in your closed car it would be very unpleasant for you too.
I personally would rather have other options for self defense. If your life was in danger and it could stun your assailant long enough to get away, even if it also caused you discomfort in the process, it would be worth it.