Remember the transmissions they use may have a real short first and second gear. So the rear gear at 3.08 may be sufficiant. You might be getting into granny gearing if you go too short on the rear. That is one use of a manual 6-speed. Spread the gears out and use short gears 1-3 and taller gears 4-6. I'm not so sure this is what is being done but it is the reason for the addition of 2 more gears for the 4 speed. This way you have the convenience of a short gear on take off with the gears spread out to taller gears on top.