When you are in the tank garage, TL can detect which tank you are, and there is a "+" button next to the tank name. Click that "+", a number will be add. Don't mine that number means, (it is just the number of vehicle you ever buy) then change another tank in garage, TL will detect another one. click "+" again. You may end up something like this in TL:
18
19
22
17
after that, save your work