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When I am typing a laboratory for physics, I tried using \overline inside SI from the siunitx package, like so:
\begin{align}
\begin{split}
s &= \frac{d}{t}\\
&= \frac{\SI{1.15}{m}}{\SI{1.43}{s}}\\
&= \SI{0.\overline{804195}}{m/s}
\end{split}
\end{align}
But it seems that it does not work when you have \overline inside SI, does anyone know how to fix this?
You can also set the number parsing off temporarily.
\documentclass{article}
\usepackage{amsmath,siunitx}
\begin{document}
\begin{align}
\begin{split}
s &= \frac{d}{t}\\
&= \frac{\SI{1.15}{m}}{\SI{1.43}{s}}\\
&= \SI[parse-numbers=false]{0.\overline{804195}}{m/s}
\end{split}
\end{align}
\end{document}
\SI, you may want to write \frac{\SI{1.15}{\meter}}{\SI{1.43}{\second}} and \SI[parse-numbers=false,per-mode=symbol]{0.\overline{804195}}{\meter\per\second}. I.e., let siunitx do the mapping from \meter and \second to the letters m and s.
literal mode of siunitx is also canonical as opposed to interpreted mode. The units here don't matter much as overline is the emphasis.
This does sort-of defeat the purpose of using siunitx, but it's possible with some box manipulation:
\documentclass{article}
\usepackage{siunitx,amsmath}% http://ctan.org/pkg/{siunitx,amsmath}
\begin{document}
\begin{align*}
s &= \frac{d}{t} \\
&= \frac{\SI{1.15}{m}}{\SI{1.43}{s}} \\
&= \rlap{$\phantom{0.}\overline{\phantom{804\,195}}$}\SI{0.804195}{m/s}
\end{align*}
\end{document}
I've pre-set an invisible 0.804\,195 with a visible \overline (only over the necessary part) using a right overlap (a zero-width, left-aligned box). After this, \SI sets things as usual, this time "underneath" the \overline so it looks appropriate.
