Two x-axes

Hi,

suppose you want to plot a distance - speed diagram that has two x-
axes:
distance in kilometers and distance in miles.

I guess i could use

set xlabel "km"
set x2label "miles"
plot [] [] "speeds.dat" using ($1):($3) ti "speed" axis x1y1 , \
            "speeds.dat" using ($2):($3) ti "speed" axis x2y1

But that produces the same two plots that are both visible in the
legend.
All I really want is the indication of the x2tics.

Is there a way to do this?
Thanks,
Andreas