I encountered the same issue. I am not really familiar with javascript, but I managed to show ‘km/m’ as expected by chaging program like below.
Around 1137 row in WP-GPX-Maps.js:
“return Math.round( tooltipItems[0].xLabel, fpt.dec ) + fpt.suf;”
to
“return Math.round( tooltipItems[0].xLabel, l_x.dec ) + l_x.suf;”