diff --git a/progressbar/widgets.py b/progressbar/widgets.py
index f5ecf61..76dde32 100644
--- a/progressbar/widgets.py
+++ b/progressbar/widgets.py
@@ -121,12 +121,12 @@ def update(self, pbar):
 class FileTransferSpeed(Widget):
     'Widget for showing the transfer speed (useful for file transfers).'
 
-    format = '%6.2f %s%s/s'
     prefixes = ' kMGTPEZY'
     __slots__ = ('unit', 'format')
 
     def __init__(self, unit='B'):
         self.unit = unit
+        self.format = '%6.2f %s%s/s'
 
     def update(self, pbar):
         'Updates the widget with the current SI prefixed speed.'