When Edgar Frank Codd invited relational databases his primary idea was to found it on
logic algebra and set theory. The rows of relational tables (aka relations) are sets. Therefore, there aren't duplicate rows nor any particular order of rows. From this point of view, "I am looking for something that would give me the speed field value for the row before" is rather irrelevant.
However, to put any meaningful signification on it, your table must already have a column
which allows such an ordering, for example date, timestamp or auto-increment numbers.
If you don't have such special column (or set of columns) within a table, it is impossible to determine the predecessor of a specific row because the today predecessor could be the successor row tomorrow (e.g. due to inserting and deleting rows in the meantime).
To cut a long story short, what's that special column of your table to get particularly ordered rows? You may also post the creating statement of your table.
If you're in one of those predicaments where cls.__private attributes just aren't enough since they can easily be accessed through inst._cls__private, and you need something a little more secure, here's ...