Big, clumsy, and terrifying;
The flamethrower (and its big brother) produces temperatures capable of melting steel in seconds. Due to the extreme fuel requirements, the flamethrower can only muster a limited number of shots before running dry. You're also increasing the risk of your own vehicle exploding when you take damage.
When any flame producing weapon takes damage or the vehicle catches fire, the vehicle will operate normally until one of two thing happen.
(Best case scenario) Your car drives through the flames, shooting your target(s) and extinguishes itself...... (Bad news) Your car drives through the flames shooting normally then BOOM!.... Say goodbye to your car, the gangers in the car and possibly any other gangers or cars within 2 car lengths of your detonation.
The resulting explosion will be powerful enough to kill your car, every one in it, AND seriously damage or kill any cars with occupants within 2 car lengths of your ill-fated vehicle.