802.11g and 802.11b in an ad-hoc network

I'm not an expert on this, but it is my understanding that any 802.11b devices accessing 802.11g/b devices in a mixed mode (on the same channel) will force ALL devices to communicate at 802.11b speeds.

Close, but not exactly. The "b" stations are approximately 5 times slower than the "g" stations. If any "b" station is constantly streaming data, it will hog the available bandwidth. For every unit of time "g" data is being transmitted, there are 5 units of time in which the "b" data is being transmitted (roughly speaking). The "g" device may be sending at 54 Mbps when it has the chance, but it doesn't have that chance very often. The net effect is to drag the overall throughput down to the range of 11 Mbps. It only takes a few legacy devices to have this effect.

Even if the "b" stations are mostly idle, the "g" stations precede every data tranmission with a CTS message at a "b" bitrate to warn the legacy stations to stay off the air (these stations cannot detect "g" signals). These protection messages slow throughput even if the "b" stations aren't doing anything.

If you want details, see:

I believe this was true for older routers, but is not so any more.

Nope, see other post. Used to have a b and g, was 11 then, got a new card to replace the b, and now it's 54.

I`ve asked a very similar question to this as I`ve been told the max speed an Ad-hoc setup can work is 11.Mbps even if you use `g` gear - something to do with when the standard was setup.

well of course, it stands to reason if you have 2 cards and 1 is a B then the fastest speed you will get is 11Mbs. Your Network is only as fast as its slowest part.

If you Ad-Hoc 2 G cards then your speed will be 54Mbs baring distance variables and bad equipment of course.