  A subset $B$ of a group $G$ is called a difference basis of $G$ if each
element $g\in G$ can be written as the difference $g=ab^{-1}$ of some elements
$a,b\in B$. The smallest cardinality $|B|$ of a difference basis $B\subset G$
is called the difference size of $G$ and is denoted by $\Delta[G]$. The
fraction $\eth[G]:=\Delta[G]/{\sqrt{|G|}}$ is called the difference
characteristic of $G$. We prove that for every $n\in\mathbb N$ the dihedral
group $D_{2n}$ of order $2n$ has the difference characteristic
$\sqrt{2}\le\eth[D_{2n}]\leq\frac{48}{\sqrt{586}}\approx1.983$. Moreover, if
$n\ge 2\cdot 10^{15}$, then $\eth[D_{2n}]<\frac{4}{\sqrt{6}}\approx1.633$. Also
we calculate the difference sizes and characteristics of all dihedral groups of
cardinality $\le80$.
