how to set ToolTip Text in android custom seekbar?

You should have your customized textview and change the text inside onProgressChanged.

Is that enough? ==> No

You need to change the x coordinate of the textview to change it's place to be compatible with seekbar place.

Code demonstrate that:

@Override
public void onProgressChanged(SeekBar seekBar, int progress,
        boolean fromUser) {

    yourTextView.setText(progress + " miles");
    // Get the thumb bound and get its left value
    int x = seekBar.getThumb().getBounds().left;
    // set the left value to textview x value
    yourTextView.setX(x);
}