What kind of gas milage increase could you expect on a 4 speed manual if you went from 3.5 diff gear ratio to like 3.0?

this is for an electric vehicle with a 9" ford rear and we want maximum highway milage for the competition.....we're doing anything/everything to drop weight and increase milage.......\

how low do ford rear diff gears go? when does a lower diff stop helping highway fuel economy?