I ran a similar test after benchmarking avg 40 times of WR & OL positions (based on 10 yrs of draft data) to ratings (avg WR 40 time came out to exactly 90 spd so good job Madden devs on that one).
From there I timed different 40-yd runs across different thresholds.
I concluded 75-80 was the sweet spot for realistic speeds.