I don't know for sure which are the most accurate, but the most consistent ones I use are speedtest.net & mybroadbandspeed.co.uk
They both give reasonably similar results & appear to be consistent with their reporting.
e.g. when my connection is poor, I get poor results.
I personally prefer speedtest.net as it logs your test results over longer periods than mybroadbandspeed.co.uk, & these results can be easily exported to a csv file for really long term monitoring etc. (also recording the IP address in use at the time of the test, so showing re-syncs etc.).
Registration with speedtest.net is not required for these basic & useful facilities.
The TBB speed tester can, on occasions, appear to give poor results, even when a user's connection is otherwise "O.K."
The usual BT performance/speed tester ALWAYS produces slightly more pessimistic results, but apart from the occasions when it experiences obvious "problems", is also quite consistent.
It does take much longer to conduct & can only be conducted once every hour, so not so good for instantly monitoring what happens if you should choose to reboot/re-sync the modem.
To me, consistency is the most important aspect & switching between speed testers just to get a better result doesn't assist at all.
Always using the same speed tester, regardless of whether results are ALWAYS pessimistic or ALWAYS optimistic, at least demonstrates a definite pattern to your connection's real-life performance (subject to any contention / traffic management for various types of traffic etc).
So, in addition to sticking with the same speed tester, I believe that speed tests should always be conducted at the same time of day/evening/night to truly monitor a connection's performance over as prolonged a period as possible.
Again, for consistency reasons, timing the download of a fixed sized file at regular times of day appears to confirm connection performance patterns.
One of BT's URLs for just this purpose can be found at this link:-
BT Test Files
The largest file is only 15MB (15,675,392 bytes on disk), which was probably ideal for much slower ADSL1 connections, but it IS a constant to use even over FTTC connections.
e.g. For my own connection from when it was "problem-free" (speed tests & ability to download at up to 33 Mb), I could usually download the 15MB file in 3 seconds.
It currently takes 5 seconds (pretty much at any time of day or night).
When my connection was variously performing even worse than it is now, it could take up to 14 seconds.
I accept that the "accuracy" of the timings is a bit loose (whole seconds only), but again, over a prolonged period a pattern emerges, & a sudden difference really does highlight "problems".
I would be curious to see timings for downloading the 15 Mb file from other users, especially those whoose speed tests are around 33 Mb.
The above is purely my own opinion, so please feel free to agree/dsagree accordingly.
P.S. I have no idea what the test file (largedownload.me) contains, but suspect it is just typical & consistent "data", specifically designed for testing purposes.
Edited by Bald_Eagle1 (Wed 21-Dec-11 09:21:10)