diff --git a/opendm/tiles/gdal2tiles.py b/opendm/tiles/gdal2tiles.py
index 081c335a5..2db97bf23 100644
--- a/opendm/tiles/gdal2tiles.py
+++ b/opendm/tiles/gdal2tiles.py
@@ -209,7 +209,6 @@ def __init__(self, tileSize=256):
         self.initialResolution = 2 * math.pi * 6378137 / self.tileSize
         # 156543.03392804062 for tileSize 256 pixels
         self.originShift = 2 * math.pi * 6378137 / 2.0
-        # 20037508.342789244
 
     def LatLonToMeters(self, lat, lon):
         "Converts given lat/lon in WGS84 Datum to XY in Spherical Mercator EPSG:3857"
@@ -223,10 +222,11 @@ def LatLonToMeters(self, lat, lon):
     def MetersToLatLon(self, mx, my):
         "Converts XY point from Spherical Mercator EPSG:3857 to lat/lon in WGS84 Datum"
 
-        lon = (mx / self.originShift) * 180.0
-        lat = (my / self.originShift) * 180.0
-
-        lat = 180 / math.pi * (2 * math.atan(math.exp(lat * math.pi / 180.0)) - math.pi / 2.0)
+        inv_originShift = 180.0 / self.originShift
+        lon = mx * inv_originShift
+        lat = my * inv_originShift
+        pi = math.pi
+        lat = 180.0 / pi * (2.0 * math.atan(math.exp(lat * pi / 180.0)) - pi / 2.0)
         return lat, lon
 
     def PixelsToMeters(self, px, py, zoom):
@@ -395,12 +395,15 @@ def Resolution(self, zoom):
     def ZoomForPixelSize(self, pixelSize):
         "Maximal scaledown zoom of the pyramid closest to the pixelSize."
 
+        resFact = self.resFact
+        resolution = resFact
         for i in range(MAXZOOMLEVEL):
-            if pixelSize > self.Resolution(i):
+            if pixelSize > resolution:
                 if i != 0:
                     return i-1
                 else:
                     return 0    # We don't want to scale up
+            resolution /= 2
 
     def TileBounds(self, tx, ty, zoom):
         "Returns bounds of the given tile"