As a rule of thumb:
Given a broadband line speed, dividing by 5 and taking off 25% is a reasonable estimate of the maximum likely data download speeds (in bytes of data).
My connection: Rate from ISP 10 mb/s = (10/5)-25% = 1.75 mb/s
TCP has an overhead in transmission that can be about 5-10%, but ATM overhead has a 15% overhead. So you can expect to lose up to 25% of your purchased speed at when counting application data transfer rate.
There are additional factors affecting the throughput results. There are applications running that usually take up a small amount of bandwidth to communicate with their servers, therefore only the remaining bandwidth is available for your downloads.
The Operating System TCP settings will also affect the results because it is usually not optimized. Visit http://www.dslreports.com for further information on optimizing your TCP settings.