The following items may affect the accuracy of a DSL speed test:
Computers which are connected to routers, switches, or using wireless access points will have inherently slower transfer speeds compared to computers which are directly connected to a DSL modem.
Computers which have malicious software installed (i.e. viruses, spyware, or adware) may experience slow transfer speeds, as software of this type tends to drastically slow down your broadband connection. If you suspect that your PC contains malicious software, we recommend that you connect your computer directly to the DSL modem and scan for any malicious software. After this has been completed, please perform your speed testing again.
In order for bandwidth speed tests to be accurate, you must be located as close to the test server as possible. If you are using a bandwidth speed test which is hosted on a webpage in California, for example, the test packets must travel a long way to reach you and this and will obviously skew the results of the test.
Finally, normal data overhead from DSL modem transmissions can sometimes use up to 10% or more of your modem's available bandwidth. This data overhead is NOT reflected in download speed tests.