Given that the days are the independent variable, which dependent variable has a constant rate of change

Consider the change in average speed with respect to days.

Change in days  Change in avg. speed
   
4 - 3 = 1                58 - 55 = 3
5 - 4 = 1               63 - 58 = 5
The change in average speed is not constant with respect to change in days.
There is no need for a further check.

Consider the change in distance with respect to days.
Change in days  Change in distance
 
4 - 3 = 1                  660 - 495 = 165
5 - 4 = 1                 825 - 660 = 165
6 - 5 = 1                 990 - 825 = 165
7 - 6 = 1                1155 - 990 = 165
The change in distance is constant with respect to change in days.

The dependent variable distance has a constant rate of change.