Future editable select list option?
Instead of using the list field type, I'd suggest you to use taxonomy and term reference field. That way you can always update the referenced term independently, add new terms, delete terms etc.


Create a vocabulary for the field, for example, depreciation
Add as many terms as you want to have in the select list
Add a new field in your content type, and as field type select Term reference; as widget select Select list
In the field options you should assign the newly created vocabulary (e.g. depreciation) to that field


UPDATE:

I assume that you have


a content type called 'car'
a vocabulary called 'model'


You should create a new vocabulary for each list field that you need to make future editable/expandable. If I understood correctly, you want to have the 'depraciation rate' editable, so you should create a new vocabulary called depreciation_rate


Go to admin/structure/taxonomy/add
Name it 'Depreciation Rate' or whatever you like
Add as many terms as you want to have in your 'select list' field. The terms will be listed as options in that field.


Then, you need to create a field to reference this vocabulary on the car content type if depreciation rate is different for every car (or on the model vocabulary if the depreciation rate is different for every model).


Go to admin/structure/types/manage/car/fields
Add new field, name it 'Depraciation Rate' with the machine name 'field_depreciation_name', or whatever you like.
As field type select 'Term reference' and as widget 'Select list'
On the field settings, assign the vocabulary you created.