Yeah its the newest popular thing in FPS games. Exploding barrels (or cars in this case) to liven up the combat.
I agree with everything said so far. Every car in fallout was raided and scavenged and broken down for building materials (junktown

). There is no reason, even in a large city, you would have exploding cars. I mean yes putting in somthing that explodes is a good idea but not the cars! Also the mushroom cloud from the car gives it a far more powerful appearance, let down by its small power in comparison.
Perhaps you could have it so if you shoot a car it just bursts into flames or has a small explosion (like a grenade or somthing) from the engine block. Thats slightly more believable but even then I think its pushing it. All cars in Fallout (bar a RARE few) are scrap. Not exploding, not running, not moving, not stable enough to stand on the roof without it falling in -scrap.
That said you never know, maybe the way Beth has done it the 'West side' is just bloody rough, while the 'East side' is jolly and care free with running cars every ten meters. TBH it all depends on exacution, maybe Beth will pull it off but if you just say it like it is "Nuclear exploding cars everywhere" it does sound stupid and not somthing which will compliment the game.