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

+1d8 for every 3 additional speed

ascia-elf