Online speed tests typically measure the amount of data reaching the device you are using, during a certain period of time. What is measured is therefore what kind of performance you achieve then and there, not what speed your subscription says you have.
For a computer or other device with a wired connection to the internet, such measurements are relatively efficient and reliable, because the connection itself will usually provide stable and predictable performance. To measure that you get the speed you are paying for, such a speedometer is a good broadband test on a wired connection.
With a wireless connection, however, the actual performance of the network is affected by a number of factors other than the speed that is delivered to your home. The coverage for the PC or device you use to measure with will affect the result. Even the coverage of other devices on the same wireless network may affect the results. If you're wondering what other devices have to do with anything, see Bad apple: How a Single Device With Poor Coverage Can Spoil Your Network.
To summarize, a test of the speed of a wireless connection tells you only what speed you have achieved
In other words, this is not a good measure of how well wireless works overall in the home. Even if you get a good result on a speed test, the wireless experience may be neither fast nor stable over time or elsewhere in your home.
If you would like to test your wireless network, the question is what you want to accomplish and what you really want to test:
This does not mean that one should never measure Wi-Fi speed, just to take it with a pinch of salt and combine it with other tools.
A speed test of the wireless can provide useful information along with other tools and measures. The best thing is to combine the speedometer with a heat map, as mentioned above.
Choose a speed test that has a fast internet connection itself and that is located as close to you geographically as possible. It may be a good idea to ask your Internet service provider (ISP) whether they have a separate tool or service that you should use when measuring.
If you choose to use a speedometer, it is important to remember the following when measuring the speed:
Article by Jan Pedro Tumusok and Jorunn D. Newth