This has been addressed before, in the T20 errata forum (i think)...but I don't have time to search for the exact thread at the moment...in any case it is something like...
if the hull in fully streamlined it can do a maximum of 2G in atmosphere...if partially, it can do a max of 1G. I'f it is an Airframe hull it can do a maximujm of it's full G rating.
to find the speeds for the various G ratings see the Airframe Atmospheric Speed table at the begining of the Design section.
Hope that helps...BTW, if you were wondering, none (that I've found anyway) of the Standard designs has the correct Atmos Speeds except the SDB (IIRC)
-Roger