You can only gain max speed by losing weight. 1 piont of speed around every 7 pounds lost.

If you draft a WR with 97 speed at 185 pounds and make him a RB, he will gain weight and approach 217 pounds (standard weight for RB). Thats 32 pounds divided by 7 which equals 4.57. Putting him at around 92 speed at RB.

If you draft a 86 speed TE at 265 pounds and convert him to RB, he will lose weight to approach 217 pounds which equals 48 pounds. That's 48/7= 6.85, so he will gain 7 speed on his max and end up around 93 max speed.

Of course, you can do what all the experts are doing and make everyone, RB/FB/TE a WR so they all lose weight and win that way but its frowned upon by most but the owners that do it win and don't seem to care if it seems wrong to the majority.