Cars have a natural class it fits into, then you can add or subtract weight to move up or down one class. But the car needs to be complete to meet rules. Some cars can't legally remove enough weight to get to the next lighter class. They only allow 170 lbs for driver. So if driver is 300 then you have a 130 lb disadvantage from the start.
The number starting with 3 means the driver lives in division 3. Driver can claim a different division but the driver number will not change.