How do I change the \mathtt font with mathspec?

Question

I have a document where the main font is Gotham Narrow Book but the numbers are set in Gotham Narrow Light, and this is set with commands provided by the mathspec package. I'd like to typeset a table where the digits come from a monospace font so that they'll line up better. But my invocation of \setmathtt has no effect.

Here is my sample document:

\documentclass{article}
\usepackage{mathspec}
\setmainfont[BoldFont=Gotham Narrow Medium]{Gotham Narrow Book}
\setmathrm{Gotham Book}
\setmathfont(Latin){Gotham Narrow Book}
\setmathfont(Digits){Gotham Narrow Light}
%:FIXME seems to have no effect
\setmathtt{Courier} 
\begin{document}
\begin{tabular}{cc}
 Normal         & TT \\
 $123456789$ & $\mathtt{123456789}$ \\
 $111111111$ & $\mathtt{111111111}$
\end{tabular}
\end{document}

Does \setmathfont(Digits){Gotham Narrow Light} preclude \setmathtt{Courier}?

Answer 1

There seems to be a bug in mathspec; if I ask for

\showthe\mathcode`1

I get 1841, i.e., "0731. It seems that 7 is the math group for Gotham Narrow Light, so the mathcode should be "7731. Indeed, if I say

\mathcode`\0=\numexpr\mathcode`0+"7000\relax
\mathcode`\1=\numexpr\mathcode`1+"7000\relax
\mathcode`\2=\numexpr\mathcode`2+"7000\relax
\mathcode`\3=\numexpr\mathcode`3+"7000\relax
\mathcode`\4=\numexpr\mathcode`4+"7000\relax
\mathcode`\5=\numexpr\mathcode`5+"7000\relax
\mathcode`\6=\numexpr\mathcode`6+"7000\relax
\mathcode`\7=\numexpr\mathcode`7+"7000\relax
\mathcode`\8=\numexpr\mathcode`8+"7000\relax
\mathcode`\9=\numexpr\mathcode`9+"7000\relax

I get the desired behavior.

Answer 2

It works with \setmathtt{Courier New} and something like \mathtt{f(x)} because the digits are set with Gotham.