It already has a similar gear ratio in 5th to a car in terms of mph per 1000rpm.
The engine is likely to be more economical at 90mph with its current gearing than it would be if it was doing 1500rpm at a much greater throttle opening.
With the current gearing you can run the engine down to idle in top gear and accelerate back up to top speed without any bucking or jumping. Although it does vibrate a bit till it gets up past 1300rpm or so.
Adding a 6th gear that tall would result in a huge jump between fifth and sixth which would make for snatchy horrible downshifts.
Curing the above problem would result in ending up with excessively tall lower ratios to maintain a respectable spread between the ratios.
The rocket gearbox is pretty agricultural as it is. Do you really want another shift in there?
A 5 speed transmission suits this kind of bike very well giving it a fairly relaxed nature.
With a 6 speed transmission you will end up dancing up and down the box on a regular basis, spoiling the relaxed nature of the bike.
A sixth gear that tall will be a mega overdrive, you may end up in a situation where there wont be enough power to pull that ratio up a hill or into a headwind, requiring a downshift. There is a LOT of wind resistance at 90mph on a naked bike.
The rocket gearing already allows it to hit a theoretical 180mph or so in top.
Gearing is a mysterious thing at times, an example is I used to have a yamaha YBR 125 which came geared for something like 86mph in top gear from the factory. The result is that any kind of a headwind and you were screaming the nuts off it in 4th gear as it didnt have the power to pull 5th and you would be down to 60mph. Any sort of hill and the speed would start dropping off also. Fuel economy was about 80mpg. Going down a few teeth on the front sprocket changed the gearing to a more respectable 74mph at redline in 5th. It was then able to pull 5th gear uphill and into headwinds. It would hold its speed better on hills. It would do 65mph into a headwind instead of 60mph. The shorter gearing allowed it to top out at 72mph on the flat instead of the 67ish before from having too high a top gear. Acceleration was much improved in all the gears. And the fuel economy actually improved by 10mpg.
Another example is a cbr600 I used to own. It was geared for a theoretical speed of 170mph in 6th gear at redline. Its top speed was actually about 155mph 1000rpm short of redline. Going down a tooth on the front sprocket made a huge difference to the acceleration through the gears. Its top speed increased to 160mph, this time about 100rpm short of redline. And also, fuel economy improved too.
Taller gearing isnt always better, in most cases slightly shorter gearing improves bikes performance in several ways. Most are overgeared from the factory to ensure they can never ever reach a high enough speed in top gear to be overrevved. And noise and emissions testing is done at certain speeds and engine rpm. Dropping the rpm for a given speed is of an advantage here for manufacturers to get through these tests to allow the bike to be approved for mass production.
