To get your approximate actual download speeds, you should take the speed number your service gives you, divide by 8, and subtract 20% of the result. This is because they are advertising in Kilobits/s rather than the Kilobytes per second that are actually used outside sneaky advertising. There are 8 bits in a byte. The 20% is because that is around the average amount the signal will degrade while traveling to your house over copper wires. Of course, the source of your download can also affect your speed. This is just a ballpark figure that should give you an idea of around what to expect, not an exact number.
Example:
3 Mb/s (3000 kb/s) Internet -> 3000 / 8 = 375
375 * .2 = 75
375 - 75 = 300 Kilobytes/second