1-3: 
1 + 1d6
4-6: 
1 + 1d8
7-10: 
1 + 1d10
11-13: 
1 + 1d12
14-17: 
1 + 2d8
18-20: 
1 + 1d20
21 +: 
1 + 1d20 + 1d6;

+1d6 for every 2 additional speed

xodian