I'm looking at the GURPS Basic Set - 3rd edition Revised. Page 138 - Vehicular Comabat
There are two main types of combat from vehicles. The first is with personal weapons (eg firing from a getaway car). This will happen from time to time in many campaigns; these rules cover it adequately.
Other combats use weapons actually mounted on the vehicle (tanks, planes, autoduelling, etc). The drivers either use their own Gunner skill, or carry gunners as passengers. This is more complex, and different for every type of vehicle. It is covered in detail in GURPS VEHICLES
Some vehicles get a dodge (like a fighter jet) and some don't (like a tank) instead relying on armor for apassive defense.
For shots AT A CAR, that penetrate, you roll 2d6.
2 - Hits drive
3-4 Hits passenger (randomly assigned)
5 windshield shattered, -3 to driving rolls until fixed
6-8 exits harmlessly
9- tire hit, reduce speed by 10mph
10-11 cargo damaged (DM ass pull)
12 - Engine crippled, car stops
If you crit a car, roll 1d6; on a 1-3 the driver gets hit, on a 4-6 the engine is crippled.
The mounted weapons are extremely dangerous - they can kill people very easily since they're designed to damage vehicles.