No, there's correlation, but just because one guy runs a 4.5 and another runs a 4.55 doesn't mean the 4.5 should get a higher speed in Madden. It depends on how they play on the field.
The biggest issue is guys will vary a good .1 to .25 based on track conditions, timing method, weather, and what the dude had for breakfast. A guy will run a 4.55 one day come back and run a 4.40 the next. 4.55 is considered average, 4.40 is fast... do you really want to base their speed rating on a test so fickle?
Bill Walsh's philosophy was that 40 time was only valuable at the extremes- a 4.2 is fast, a 4.8 is slow. Otherwise, glance at it, know he's fast enough and know whether or not he's a burner... after that, the 40 should be put aside. It's just too unreliable, and it's a poor measure of how well a player will perform on the field.