[AnnaSB]

pawn Code:

OnGameModeInit()
{
    // ...
    for (new i = 0; i < MAX_VEHICLES; i++) Gas[i] = 1.0 * (random(65) + 15);
    // ...
}

OnVehicleSpawn(vehicleid)
{
    Gas[vehicleid] = 1.0 * (random(65) + 15);
    // ...
}

Edit: You also need a timer to respawn all unoccupied vehicles with an interval (ex. 60000ms = 10m) to prevent out-of-fuel vehicles from being abandoned forever; I'll leave that to yourself.

Hope I helped ^^