Vehicles have a size category ranging from 1 to 4. A vehicle's size determines its seating capacity, storage capacity, its hull points, and its fuel die.
Vehicles have the three physical ability scores: Strength, Dexterity, and Constitution. (Vehicles with some kind of artificial intelligence, like KITT from Knight Rider, can have the three mental ability scores.) Generate vehicle ability scores like you would a characters.
Seating and Storage Capacity
The former is the number of normal sized creatures the vehicle can transport; the latter is the amount of stuff a vehicle can transport—represented by slots. It's possible to break both of these capacities; however, breaking one calls for a CON check during travel. On a failure, the axle breaks from the strain and the vehicle is left inoperable. Breaking both capacities gives this CON check disadvantage.
After 25 miles of driving, roll the vehicle's fuel die. If the result is an odd number, the vehicle uses up a portion of its fuel and the fuel die decreases by a step. A fuel canister can be emptied into a vehicle to increase its fuel die by one step up to its max. A fuel canister takes up two inventory slots.
Vehicles have hull points and 1 hull point is equivalent to 10 hit points of damage. This means that a damage roll has to be 10+ to damage a vehicle. Unlike hit points, hull points do not recover naturally and require repairs. It takes a day of work to repair 1 hull point if materials are available.
Vehicle Armor Class
Small vehicles have less coverage but are fast, while larger vehicles are slow but have more coverage. Therefore a vehicle's size category also determines its AC.
To generate a vehicle on the fly:
1) Roll a d4 to determine its size category.
2) Generate its ability scores.
3) ... and that's it really!