I assume these numbers are "mbps", i.e., megabits per second.
We know your ISP is providing 16-22mbps under ideal conditions (wire). But since wireless G performance can vary greatly, it’s entirely possible you’ll get much less. I will say that 2.6-5.0mbps seems exceptionally slow. I’m getting about 15mbps w/ wireless G, while my ISP is providing 10mbps. So both my wired and wireless adapters show the same performance since neither can’t saturate the Internet connection. All I can say for sure is that your ISP is definitely not the problem. Wire proves it.
The only thing that immediately comes to mind is perhaps your wireless router is configured for mixed mode (B+G). This can have a significant negative impact on wireless performance. Unless you have wireless B clients, make sure your wireless router is configured for G-only.