\documentclass{article}

\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}

\usepackage[sfdefault]{classico}
\usepackage[EULERGREEK]{sansmath}\sansmath

\DeclareSymbolFont{mysymbols}{T1}{cmr}{m}{n}
\DeclareMathSymbol{=}{\mathrel}{mysymbols}{`=}

% \catcode`==13 \def ={\ensuremath{=}}

\begin{document}


\begin{center}
text: 1 2 3 = 4 5 6
\end{center}

\begin{center}
math:  $ 1 2 3 = 4 5 6$
\end{center}

\begin{center}
  arrow: $\Longrightarrow$
\end{center}

\end{document}