I have an imported mountain bike that I want to get it back on the road, it has a broken rear axle and I will be replacing it with a new wheel, but I am trying to learn more about Bike mechanical too.

The sticker says Shimano 18 speed but I am not sure why there is a 7 speed freewheel in the back. I identified the tool to use to transfer over the Shimano freewheel should be Park Tool FR1 (12 spline).. Do they sometimes put a 7 speed freewheel even though the shifter is only for 6 speeds (3x18).

I measured the frame to be 5 5/16" which translate to 135mm, and I will be looking for a 135mm spacing wheel.