app.js

var fs = require('fs');
var speed = require('./speedometer');
var write = fs.createWriteStream('./dump.bin');
var read = fs.createReadStream('/dev/urandom');

var measure = speed(write, 1000);
measure.on('speed', function(bytes){
    console.log(bytes + ' bytes per seconds');
});

read.pipe(write);

speedometer.js

var events = require('events');

module.exports = function(writable, interval){
    var emitter = new events.EventEmitter();
    var stop = false;
    var lastBytes = 0;

    var sendCalc = function(bytes){
        emitter.emit('speed', bytes - lastBytes);
        lastBytes = bytes;
    };

    (function loop(){
        if(stop)
            return;

        var newBytes = writable.bytesWritten;
        sendCalc(newBytes);

        setTimeout(loop, interval);
    })();

    writable.on('finished', function(){
        stop = true;
        sendCalc(writable.bytesWritten);
    });

    return emitter;
};