diff --git a/youtube_dl/downloader/common.py b/youtube_dl/downloader/common.py
index c0af50c59..6e44c88c9 100644
--- a/youtube_dl/downloader/common.py
+++ b/youtube_dl/downloader/common.py
@@ -80,6 +80,8 @@ class FileDownloader(object):
     def calc_eta(start, now, total, current):
         if total is None:
             return None
+        if now is None:
+            now = time.time()
         dif = now - start
         if current == 0 or dif < 0.001:  # One millisecond
             return None
@@ -146,18 +148,19 @@ class FileDownloader(object):
     def report_error(self, *args, **kargs):
         self.ydl.report_error(*args, **kargs)
 
-    def slow_down(self, start_time, byte_counter):
+    def slow_down(self, start_time, now, byte_counter):
         """Sleep if the download speed is over the rate limit."""
         rate_limit = self.params.get('ratelimit', None)
         if rate_limit is None or byte_counter == 0:
             return
-        now = time.time()
+        if now is None:
+            now = time.time()
         elapsed = now - start_time
         if elapsed <= 0.0:
             return
         speed = float(byte_counter) / elapsed
         if speed > rate_limit:
-            time.sleep((byte_counter - rate_limit * (now - start_time)) / rate_limit)
+            time.sleep(max((byte_counter / rate_limit) - elapsed, 0))
 
     def temp_name(self, filename):
         """Returns a temporary filename for the given filename."""
diff --git a/youtube_dl/downloader/http.py b/youtube_dl/downloader/http.py
index 8491cee8a..224962e86 100644
--- a/youtube_dl/downloader/http.py
+++ b/youtube_dl/downloader/http.py
@@ -136,16 +136,21 @@ class HttpFD(FileDownloader):
         byte_counter = 0 + resume_len
         block_size = self.params.get('buffersize', 1024)
         start = time.time()
+
+        # measure time over whole while-loop, so slow_down() and best_block_size() work together properly
+        now = None  # needed for slow_down() in the first loop run
+        before = start  # start measuring
         while True:
+
             # Download and write
-            before = time.time()
             data_block = data.read(block_size if not is_test else min(block_size, data_len - byte_counter))
-            after = time.time()
+            byte_counter += len(data_block)
+
+            # exit loop when download is finished
             if len(data_block) == 0:
                 break
-            byte_counter += len(data_block)
 
-            # Open file just in time
+            # Open destination file just in time
             if stream is None:
                 try:
                     (stream, tmpfilename) = sanitize_open(tmpfilename, open_mode)
@@ -161,11 +166,22 @@ class HttpFD(FileDownloader):
                 self.to_stderr('\n')
                 self.report_error('unable to write data: %s' % str(err))
                 return False
+
+            # Apply rate limit
+            self.slow_down(start, now, byte_counter - resume_len)
+
+            # end measuring of one loop run
+            now = time.time()
+            after = now
+
+            # Adjust block size
             if not self.params.get('noresizebuffer', False):
                 block_size = self.best_block_size(after - before, len(data_block))
 
+            before = after
+
             # Progress message
-            speed = self.calc_speed(start, time.time(), byte_counter - resume_len)
+            speed = self.calc_speed(start, now, byte_counter - resume_len)
             if data_len is None:
                 eta = percent = None
             else:
@@ -186,9 +202,6 @@ class HttpFD(FileDownloader):
             if is_test and byte_counter == data_len:
                 break
 
-            # Apply rate limit
-            self.slow_down(start, byte_counter - resume_len)
-
         if stream is None:
             self.to_stderr('\n')
             self.report_error('Did not get any data blocks')