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

+1d8 for every 2 additional speed

rover-elf