Loewner driving functions encode simple curves in 2-dimensional simply connected domains by real-valued functions. We prove that the Loewner driving function of a $C^{1,β}$ curve (differentiable parametrization with $β$-Hölder continuous derivative) is in the class $C^{1,β-1/2}$ if $1/2<β\leq 1$, and in the class $C^{0,β+ 1/2}$ if $0 \leq β\leq 1/2$. This is the converse of a result of Carto Wong and is optimal. We also introduce the Loewner energy of a rooted planar loop and use our regularity result to show the independence of this energy from the basepoint.