Dimensional formula for speed in SI is                          

The dimensional formula for speed in S.I is $\left[{\text{L}}^1{\text{T}}^{-1}\right]$

Because we know speed $=\frac{\text{distance}}{\text{time}}=\frac{\text{L}}{\text{T}}\Rightarrow {\text{LT}}^{-1}$

So dimensions of speed in length is 1 and in time is $-1$

Hence dimensional formula for speed $\left[{\text{L}}^{\text{1}}{\text{T}}^{-1}\right]$