The 54Mbps is the theoretical max limit for 801.11g. It is using the 2.4 - 2.4835GHz radio frequency range. This is the so call "free" frequency range and is not owned by anyone. Most wireless equipments (including electronics equipment) use this frequency range so it can be quite crowded.
It is impossible to reach or substain transfer rate at 54Mbps as there are limited channel (11 or 13 depend on the router you use) and these are shared. By default, most older home router will use the first channel (2.412GHz). Do not know about the current generation of home routers. You can manually changed that in the router setup page.
There are other factors that could limit the transfer speed.
- drivers
- type of encryption
- number of wireless devices around your place
- single or mixed mode
- the number of devices using the same channel as you.
The transfer rate for my home varies from 24 - 54Mbps. I am able to obtain constant 24Mpbs transfer rate.
Maybe you might want to "scan" your area to check the number of wireless access points.
I am no expert in wireless networking and these are just my 2 cents worth of sharing.
Please correct me if there are any mistake mention above.