From the iwconfig man page; emphasis added:
rate/bit[rate]
For cards supporting multiple bit rates, set the bit-rate in b/s. The bit-rate is the speed at which bits are transmitted over the medium, the user speed of the link is lower due to medium sharing and various overhead.
Not all NICs/drivers can support setting bitrate via iwconfig, and will operate at their default speed for the negotiated connection regardless of whatever iwconfig has to say about it.
Also, the man page says little about the 'fixed' token, but it looks like you want to use 'auto' instead of 'fixed'.