%% 
%%  A DANTE-Edition example
%% 
%%  Beispiel 02-26-4 auf Seite 104.
%% 
%%  Copyright (C) 2010 H. Voss
%% 
%%  It may be distributed and/or modified under the conditions
%%  of the LaTeX Project Public License, either version 1.3
%%  of this license or (at your option) any later version.
%% 
%%  See http://www.latex-project.org/lppl.txt for details.
%% 

%%Run also: >>  << 
% Show page(s) 1

\documentclass[]{article}
\pagestyle{empty}
\setlength\textwidth{170.59192pt}
\usepackage[utf8]{inputenc}

\usepackage[expproduct=\cdot]{siunitx}
\usepackage{booktabs}
\sisetup{tabformat=1.3,tabnumalign=centre}

\begin{document}
\begin{tabular}{%
  S
  S[tabautofit]
  S[tabautofit,tabnumalign=centredecimal]}\toprule
\emph{Spalte} & \emph{Spalte}
  & \emph{Spalte} \\\midrule
1.2   & 1.2    & 1.2  \\
1.234 & 1.2345 & 1.234\\
\bottomrule
\end{tabular}
\end{document}