Thanks for getting your code into code tags.
You are doing all your calculations from a starting time and starting position, rather than taking the changes between readings. In essence you get an average velocity from the beginning rather than an instantaneous snap shot of the last 12.5 ms.
Is that what you want?
Other comments:
You are better off to keep all the math using the analogRead() values 0-1023 rather than convert into voltage.
The map function does not return a float.
https://www.arduino.cc/en/Reference/Map