I don't think there should be max or mins either, and there aren't any maximums or minimums in what I ended up with. There are definitely specific ranges in the test data I used, even though it was 1300 players. But the equations I derived from them don't have any set ranges.

One reason I combined different positions into the Offensive Backfield, Receivers, Trenches and Back 7 categories is that combining fast players with slightly slower players (e.g. HBs with QBs, WRs with TEs, DL with OL, DBs with LBs) allowed me to get two sets of max/mins. The resulting equation stretches much better across both than it originally did across just one. Cornerback was the best example of this. In the original data set, you would have been hard pressed to get a CB lower than 85 SPD. That was based on the data I had, which had very, very few CBs with slow times and low SPD ratings.

In comparison, the LBs were, largely, slower. So combining the two gave me a range that, for a 4.29 to a 5.27, ranges from 94 to 72, rather than 92 to 88 (the original DB range).

SPD does mean the same for each player. Just, according to Donny Moore, certain positions have ranges for SPD that the players won't go outside of. In practice, this isn't completely a bad thing. I mean, are you really going to find a receiver or DB that runs a 5.00+ 40? No way. So there's a reasonable cutoff point that you can use.

The one big problem with that is that the range for linemen appears horribly low.