My "always" CCW handgun is my S&W Model 37 Airweight snubbie . . . ALWAYS in my left front pocket (I'm a lefty). It is easier for left-handed drivers to get to their pocket revolver than you righties.
That being said . . .
I view my defensive situations on the road, and at home, as entirely different scenarios. When I get in the car, like others have mentioned, my "always" gun becomes the weaker-powered BUG. Same at home!
Thus, when I get in the car, my main handgun is a .45ACP with plenty of reserve ammo. Depending on the day or the mood, it may be a high capacity Para . . . or my little Kimber Ultra CDP.
However, it is usually my chopped barrel 25-2 with a tray loaded with eight moonclips containing various types of bullets for various situations . . . hardball, some really hot ultra-lightweight 117 gn. Aguilla hollowpoints (both "good" against doors and glass) . . . and handloaded Speer flying ashtrays, Golden Sabre and/or Hydrashok for "soft threats."
As a car gun, I much prefer a powerful revolver rather than a bottom feeder forcefully ejecting spent cartridges into the interior of the car in the case it is needed. A .45ACP revolver though . . . is PERFECT!