I think Sean's overdrive option changes the 4th and 5th gearsets to numerically-lower ratios from other Yamaha V-4 gearboxes. It's for more-relaxed cruising, lowering your rpm's.
No, no 6th gear. The bike makes 74 ft/lb torque, plenty to get you down the road. Think about what a 6th gear would require: a new set of gears, a new shift drum, shift forks, there isn't room in the gearbox for that. Many-gears are what low-torque, small-displacement bikes used to give them the ability to stay in their power zone. Look at the GP bikes of the 1960's/'70's, where they had up-to
18 gears, because their rpm range powerband was so-narrow. Honda famously had 22,000+ rpm engines for their small displacement roadracing bikes. Since small displacement engines don't produce much torque, their power comes from rpm's.
A Venture final drive (an easy mod) or the 4th/5th gear swap (case-splitting) is how to lower cruising rpms.
Some interesting reading on GP bikes from Honda's ascendancy.
https://www.cyclenews.com/2016/08/article/tempest-in-a-teacup/
One interesting point is their 1965 RC115 50cc two-stroke campaigned in Grand Prix had all-those gears (18!), while the next-year's four-stroke RC116 50cc engine made power across a wider powerband, and allowed them to use only a 9-speed gearbox. The four-stroke 50cc DOHC twin weighed a feathery 110 lbs dry. One way to go-fast is to lower power consumed by internal friction. You can imagine the difference gained there, from only having to rotate 9 gearsets compared-to 18 gearsets. An interesting confirmation of this, and Honda's drive for innovation pushing the boundaries of optimizing mechanical drag reduction, was seen in their 6 cyl Grand Prix engines, (one being the RC166, DOHC 250cc transverse inline-six cyl) where the crank bearings were different sizes for the journals, producing reductions in component weight, reducing friction, and minimizing the need for heavier components, as the loads imposed on the engine benefitted from the reduction in weight. Also, the package could be made-smaller, and less-aero drag is an easy way to make what horsepower you produce get you through the air faster, using the benefits of laminar flow. Here's an article highlighting the benefits of laminar flow, from today's aviation news:
https://www.cnn.com/travel/article/celera-500l-plane/index.html
RC166 Honda GP bike:
https://petrolicious.com/articles/honda-rc166
https://petrolicious.com/articles/honda-rc166
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