My math only "checks out" if you accept and account for the additional assumption I made there: that the datapoints provided in the question have been rounded or were low resolution from the get-go.

The motivation behind this assumption is twofold: the numbers in the question are awfully whole (atypical for any practical problem), and that just the rote derivation of it all doesn't produce very interesting results (gives you the infinite speed answer). :)

Try introducing some error terms and see how the result changes! It's pretty fun, and it's how I was able to eek out that 41 mph result in the end.