When using Network Utility, does Link Speed tell you the actual speed of the network?

Question

I'm trying to see whether a power line adapter I installed in my home network is faster accessing the internet than the WiFi from my Time Capsule.
Using Apple's Network Utility, my power line/ethernet connection reads as being 1Gb/s, which is obviously much faster than the 130 Mb/s of the WiFi:
But are those the actual speeds my iMac is receiving, or are those just what the specs of the device I'm using should be receiving (as in "ideal, what-the-manufacturer claims the speed should be")?

Answer 1

Are you sure you only want to test internet speed?
Internet speed is affected by many factors that are beyond your control - it is also likely that the internet connection is one of the slowest parts of your network.
You may be on an ISP connection that has 'contention' e.g. a shared connection across many users - their use can effect what rates you get. You will probably see different rates for the time of day too. When everyone gets home & starts watching Netflix you can see drops in speed.
Powerline could be many times faster than the internet connection, in which case you will see no improvement because the bottleneck is not the local network - it's the internet.
To truly see if the powerline adaptors are faster you should take the internet out of the equation. It's fairly simple to do if you have a device on your network that supports file sharing. Ideally you would use another Mac connected via ethernet to the network (that should remain the same in all tests). A Time Capsule could be used if you can write data to a sharepoint.
Copy a large file onto that destination & time it.
Repeat on wifi.
It's best to shutdown any background apps to be sure that other items are not hogging all the data transfer. Take many tests to get a decent average.
Divide the file size by the time taken to see a rough figure for the MB/s (MegaBytes per second) bear in mind that network transfer is usually measured in bits/second, so they won't be the same as the link speed figures.
This is generally smart enough to convert if you give it the correct units…
http://www.wolframalpha.com
e.g. http://www.wolframalpha.com/input/?i=28MB%2Fs+in+mb%2Fs
If you have multiple Macs, iPerf is a free tool to test if you are capable in the command line. JPerf is a version with a UI (needs Java installed).