You can choose how Google Earth displays units of measurements. There are two sets of options: one for Ruler tool and the other for elevation figures as displayed in the 3D viewer.
To set units of measurements used by the Ruler tool, click the Ruler icon in the toolbar. In each tab (Line, Path, Polygon and Circle), simply choose the appropriate unit and start measuring.
To set units of measurement for elevation in the 3D viewer, click Tools > Options > 3D view (on the Mac, click Google Earth > Preferences > 3D view). Under Show Elevation, choose the appropriate unit and click OK.