I mean that's pretty much how it is right now. The fastest DL in my league is an 84 speed at 270lbs. If you convert him to a 190lb DB, I would expect his speed to be around the mid 90s. It looks like the speed/weight scale is working correctly. All the fastest players are less than 200lbs and the near 300lb players max out around 80 speed.

I haven't done any of the math for this, but just thinking about it, I would think these would be the min/max speeds for each position.

QB: 0-100 (Peyton Manning to Michael Vick)
HB: 70-100
FB: 50-80
WR: 70-100
TE: 50-80
OL: 0-70
LB: 50-80
DL: 10-80
DB: 70-100
K: 0-70