Find download speed using Javascript

Question

I'm working which detect the download speed of a given system ,and the approach I took to find that is as follow

Download files of know size and then based upon the estimated time taken calculate the download speed

Here are code look

function startDownload() {
  $('div.loading').show();
  var startedAt,endedAt;
  // Size in KB
  var size = 16384;

  startedAt = (new Date().getTime());
  console.log('.....Started.....');
  // window.earlier_entries = window.performance.getEntries().length;
  var settings = {cached: false,datatype: 'html'}
  var time = (new Date().getTime());

  $.when(
          $.ajax('/sample128k_2',settings),
          $.ajax('/sample128k',settings),
          $.ajax('/sample256k',settings),
          $.ajax('/sample8192k',settings),
          $.ajax('/sample512k',settings),
          $.ajax('/sample1024k',settings),
          $.ajax('/sample2048k',settings),
          $.ajax('/sample4096k',settings),
          $.ajax('/sample8192k',settings)
        ).done(function(a1,a2,a3,a4,a5,a6,a7){
    endedAt = new Date().getTime();
    var totalTime = (endedAt - startedAt);
    console.log(totalTime);
            // find_latency();
     var totalsec = totalTime/ 1000;

    // var totalsec = (totalTime - window.latency)/ 1000;

    console.log(totalsec);
    var size_mb = (size/1024/totalsec);
    var data,identifier,speed;

    if (totalsec > 0) {
      if (size_mb >= 1) {
        data = size_mb ;
        identifier = ' MB/s';
      }
      else {
        data = size/totalsec;
        identifier = ' KB/s';
      }
      console.log(data);
      speed = (data.toFixed(2) + identifier).fixed(2);
    }
    else {
      speed = 'Too fast seem like around certainly >= 8MB/s';
    }
    $('span.speed').html(speed);
    $('div.loading').hide();
    $('div.text').show();
  })
};

$(document).ready(function(){
  $("a#start").bind('click',function(event){
    $('div.text').hide();
    event.preventDefault();
    startDownload();
  })
});

Now The code work as expected the file get downloaded but I fail to understand I see my bandwidth get listed as

But I fail to understand as to why I see huge difference(most of time) in result in

http://speedof.me/

http://jsfiddle.net/yahavbr/tEFpC/2/

BTW mine is

http://speedboom.herokuapp.com/

Reason for difference I believe One reason I can think of that difference is latency since I downloading more then one file I quite naturally that download latency in my case is more

but

-- Even when I removed the latency (only work google chrome btw) I don't see a significant difference

     function find_latency() {
      var new_entries = window.performance.getEntries().length;
      var latency = 0;
      for(var i=window.earlier_entries;i< new_entries ; i++) {
        latency += (window.performance.getEntries()[i].responseStart - window.performance.getEntries()[i].requestStart);
      }
      window.latency = latency;
       console.log(latency);
     };

But that too didn't make any difference to speed in term of result of speedof.me and other jsfiddle result

Now the question ?

-- Whom do I trust in term of exact download speed ?

-- How do better my approach ? If I can ?