Answer: The Accelerator.
Do not under any circumstances short of the zombie apocalypse, fire a weapon from an operational vehicle. The District Attorney and worse the Judge will always assume that it was road rage on your part. Yes, this includes car jackings because car jackers believe it or not, aren't honest people. They will actually lie to the police and claim they weren't planning to steal your car at all. Shocking I know.
This legal deck is completely stacked against you. Accept this as a given fact, every hand will be turned against you if you defend yourself from inside a car.
You have now been made aware of the legal ramifications.
However Cataline has visited parts of the world where firing a weapon from a vehicle is on occasion, a life preserving necessity. Tragically America will shortly be one of them.
Okay, so assuming the shit has hit the fan and there will be few if any legal consequences for your actions because the law doesn't effectively exist anymore. There is good news and bad. The good news is you are in a car. Car's have cargo capacity. You can carry stuff in a car that you wouldn't ordinarily carry on your person due to weight restrictions and frightening little old ladies. The Bad News of course is that civilization has ended and you are now starring in your own little version of the Road Warrior.
Most cars today have fold down seats that give you access to the trunk. If you don't have a car that does that...get a car that does that. The trunk is where your real gun is and you need to get to it.
About that gun.
I'm dividing this into three categories divided by ammo.
First 5.56. Your car-gun needs to be able to be wielded around inside of...you guessed it...a car. So a lot is dependent on your space available. Possible recommendation, an AR-15 carbine...if you have the room for it. Likely you won't. The modern car is small and cluttered with headrests, a steering wheel that is just great for snagging the tip of a barrel, plus wires hanging from after market sat navs as well as cradled smart phones that will work almost as well for snarling your compensator. Still, the AR has the most punch and will be the most accurate.
Next. 5.7mm; the FN-P90. Yes, the one from Stargate; SG1. It fires a smoking hot round at very high speed. The round was designed as a compromise between a 9mm and the 5.56 AR-15 round. The ammo will have to be mail ordered. You just won't find it in stores all that often. Not as long a range as a the 5.56 but you can maneuver it inside a car easily enough.
A face only a mother could love.
Honestly, it's not my first choice. At $1300 bucks it's a little pricey for what you are getting. On top that of course you are only getting the base model. Sights, spare mags etcetera will push this one past $1500. That in addition to the ammo availability issue, puts this one on the pass list for me. However if you want desperately want the Stargate gun in your car. Enjoy, its not a bad choice.
Then we get to my personal favorite. The CZ Scorpion Evo 3 S-1
You maybe experiencing an uncomfortably tight sensation in your special trousers region.
Don't worry. That's normal.
That's supposed to happen.
It's 9mm. So you won't have any trouble at all finding ammo. You will also get more punch from the nine mil because of the longer barrel. Base model MSRP is a non-painful $849 for the base model. Spare mags will run you $20 a pop.
CONSULT A LAWYER for your own state laws. Do not Google for this. Ninety percent of everything on gun boards is anecdotal but stated with total conviction. And conviction is what you are facing if you pay attention to any of these loud mouthed barracks lawyers. Pay money to an actual lawyer and listen to him, trust me it's worth it.