1-3: 
1d4 + 1
4-6: 
1d6 + 1
7-10: 
1d8 +2
11-13: 
1d10 + 2
14-17: 
1d12 + 2
18-20: 
2d8 + 3
21 +: 

1d20 + 3

+1d6+1 for every 2 additional speed

deztunian