I think there is a way to change that with the rate option of iwconfig.

Look in the man page to see if

On 12/15/06, Olive < olive001@tele2allin.be> wrote:
I have a network which would normally support the 802.11g protocol (and
a card which support it). But I always see 802.11b in a iwlist scan.
Stranger is that I see that my wireless connection support up to 54Mb/s
which is not normally possible with 802.11b. I use ndiswrapper 1.31 with
  the following card:

02:00.0 Ethernet controller: Marvell Technology Group Ltd. 88w8335
[Libertas] 802.11b/g Wireless (rev 03)

output of iwconfig

wlan0     IEEE 802.11b  ESSID:"XXXX"
           Mode:Managed  Frequency:2.462 GHz  Access Point:
           Bit Rate:54 Mb/s   Sensitivity=-200 dBm
           RTS thr:2346 B   Fragment thr:2346 B
           Power Management:off
           Link Quality:40/100  Signal level:-70 dBm  Noise level:-96 dBm
           Rx invalid nwid:0  Rx invalid crypt:0  Rx invalid frag:0
           Tx excessive retries:0  Invalid misc:0   Missed beacon:0

output of iwlist scan:
wlan0     Scan completed :
           Cell 01 - Address: 00:C0:49:E8:8F:D5
                     Protocol:IEEE 802.11b
                     Frequency:2.462 GHz (Channel 11)
                     Quality:32/100  Signal level:-75 dBm  Noise
level:-96 dBm
                     Encryption key:on
                     Bit Rates:1 Mb/s; 2 Mb/s; 5.5 Mb/s; 11 Mb/s; 18 Mb/s
                               24 Mb/s; 36 Mb/s; 54 Mb/s; 6 Mb/s; 9 Mb/s
                               12 Mb/s; 48 Mb/s


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