6 speeds don't add a gear to the top, they add a gear to the bottom. a 5 speed transmission is commonly "4 speed with overdrive". 4th gear is 1:1 (or close) which is "direct Drive". 5th gear is an "overdrive". In your common 6 speed trans, 5th gear is Direct drive (again, close or exactly 1:1) and 6th is overdrive.
What you want to really do is build your own gearing, and you can do that through lots of patience, math and effort (Most Hondas are available with LOTS of different gearings)