Data downloads and data speeds are calculated using two different measurements:
Downloads are measured in bytes: Megabytes (MB), Gigabytes (GB) and so on. Each increment is one thousand times larger than the one before it. For example, if you download a 500MB file, that's 0.5GB of your monthly data allowance.
Speed is measured in megabits per second. There are 8 bits in a byte. Therefore, if you connection speed is 100Mbps, you can download 12.5MB of data per second. If a movie you download is 1GB in size, it will take 80 seconds to download at this speed.
If you want to keep track of your usage when downloading, the speed of your connection has no bearing on how much data you are using. The important information is the size of the file being downloaded. This is what will affect your cap.