Not sure what can be the problem. Can u please see if this command is working with the expected output:

# DOWNLOAD=999999;UPLOAD=999999;PING=1;SERVER=1718; wget --referer=http://c.speedtest.net/flash/speedtest.swf --post-data="download=$DOWNLOAD&ping=$PING&upload=$UPLOAD&promo=90&startmode=flyok&recommendedserverid=$SERVER&accuracy=1&serverid=$SERVER&hash=`echo -n \"$PING-$UPLOAD-$DOWNLOAD-297aae72\" | openssl md5`" http://www.speedtest.net/api/api.php

--2012-05-11 10:23:19-- http://www.speedtest.net/api/api.php
Resolving www.speedtest.net... 74.209.160.12
Connecting to www.speedtest.net|74.209.160.12|:80... connected.
HTTP request sent, awaiting response... 200 OK

Just finished this and wanted to share this info :-) http://mindkaleidoscope.blogspot.in/2012/05/spoofing-speedtest-results-for-your-isp.html

I guess a simple expression like

int random= (rand()%18000)*5 + 10000

should do the trick where rand() is a function that returns any positive random number for C# it can be replaced with

int random= (new Random().Next()%18000)*5 + 10000