The current implementation selects CWmin value of 31 for 802.11g in infrastructure mode  if there is at least one 802.11b station associated in the BSS. In Adhoc mode, value 31 is always chosen.

Here is the relevant part in the standard (clause 9.2.12):
In the case of the Clause 19 Extended Rate PHY, the aCWmin value is dependent on the requestor's characteristic rate set. The characteristic rate set is equal to the IBSS's supported rate set when the STA is operating as a member of an IBSS. It is equal to the AP's supported rate set when the STA is associated with an AP. At all other times, it is equal to the STA's mandatory rate set. The MAC variable aCWmin is set to aCWmin(0) if the characteristic rate set includes only rates in the set 1, 2, 5.5, 11 otherwise, aCWmin is set to aCWmin(1). If the returned value for aCWmin is a scalar, then the MAC always sets the variable aCWmin to the returned scalar value of aCWmin.

I will work on a patch.