Download Speed

Hi,
I want to calculate the download speed of a file which is being downloaded
using TIdHTTP.Get.
I use OnWork event to calculate the data readed.My code seems to work but,
it is always changing 25 to 8 KBPS, 10 to 25, it is changing very fast.What
I expected is a fixed download speed(Because NetLimiter shows a fixed value
and my calculation alg. always shows different value.).Here is the code:

procedure TDownloadThread.OnWork(ASender: TObject;
  AWorkMode: TWorkMode; Const AWorkCount: Integer);

  function CalculateSpeedPerSec(curr,pre:Integer;BufLen:Integer):Double;
  begin
   if curr = pre Then
    curr := curr+1;
   Result := (BufLen / (curr-pre))*1000;
  end;

begin
  if Not Assigned(fStream) Then
    exit;

  if AWorkMode = wmRead then
  begin
    fCurrentTick := GetTickCount;
    VirtualTreeData^.SpeedPerSec :=
CalculateSpeedPerSec(fCurrentTick,fPreviousTick,AWorkCount-fPreviousWorkCount);
    VirtualTreeData^.FilePosition := AWorkCount;
    if VirtualTreeData^.FileSize <> 0 Then
     VirtualTreeData^.DownloadedPercentage := (100 * (AWorkCount)) /
VirtualTreeData^.FileSize;

    if ((fCurrentTick-fPreviousTick) >= 1000) and (fStream.Position <> 0)
Then
     Synchronize(UpdateGUI);

    fPreviousTick := fCurrentTick;
    fPreviousWorkCount := AWorkCount;
  end;
end;