It will help with your gas mileage, however it's likely to be so slight you may not notice unless you track it closely. Manual hubs are a good swap anyway, less mechanical drag, less wear on CV boots, and they are a direct swap for your ADD flanges (you do not have automatic hubs).

4x4Wire's Toyota Section
1995 FZJ80 LandCruiser - 3 locking diffs, +200k mi, JD HIR's, 285/75R16 Nitto TG's
Olde North State Cruisers