Offline
Looking through Running Back stats at Pro-Football Reference, and it seems that there is certainly some precedent for RBs slowly improving their YPC.
Just to throw a random guy out there, let's take Thomas Jones for example.
3.3, 3.4, 3.7 his first three years - then, 4.6, 4.0, 4.3, 4.1.
"I'd like to know his TD to Fumble ratio."
That's easily found. Reggie has fumbled the ball 16 times in his career. He has a total of 13 rushing touchdowns, 8 receiving touchdowns, and 4 punt return touchdowns - a grand total of 25 TDs. That's about 1.6 Touchdowns for every fumble.
Edit: Sorry PurpleBlackandGold I see you already posted some of these stats - had my window open for a while without posting!
I like the comparison to Jones. Not every back comes out of the gate like a world beater. Some take time to develop just like receivers.