I wanted to construct a 150km distance buffer around a city in North America. I changed the project co-ordinate system of the point to WGS84 Mercator North America from WGS 84 Lat/long to enable the buffer to be constructed in metres/km. The buffer was drawn on the map, but it doesn't show on top of local US neighbourhood census tract boundary files (which I changed to the same co-ordinate system). When I zoom to the full extent of the buffer layer, it appears to be different co-ordinbates to the census tracts layer. Am I using the correct co-ordinate system? I assume 'on the fly reprojection' is automatically turned on in QGIS3.