TDIMeister
Phd of TDIClub Enthusiast, Moderator at Large
There is a better way to illustrate this than mere words and that more precisely explains why .622 is the most optimal 6th gear ratio in the case where the other 5 speeds are CTN-stock. It starts with plotting a logarithmic trendline on the speed-vs-RPM points on the bottom after a gear shift:
I'm also not concerned about the 6th gear ratio in and of itself but rather in relation to the other gears, particularly 5th, which is why I post the sawtooth patterns to compare among different ratios.
A stock-CTN + 0.622 6th gear fits pretty nicely in this curve and without going through a great deal of complexity, it means that the engine revs build up progressively at the bottom of each subsequent upshift from the assumption of a constant upper RPM where the shifts take place. The engine is more likely to have a torque backup at the lower RPM after the upshift as long as the shift point does not fall much below the exponential curve. OEMs engineer their cars and choose gear ratios not out of thin air but rather following this principle. Trucks and transmissions for other vehicles may use a different strategy (e.g. a horizontal or linear progression instead of logarithmic as I've illustrated, but gear ratios for passenger cars are done this way almost without exception.
By contrast, you can see in the example of poorly chosen ratios:
Even without the exponential trend curve in place, you can imagine that in the above case, it will be very annoying to row through the gears, especially from 3rd to 4th, because the engine sounds corresponding to the RPM will be off from the expected progressiveness and then the deep RPM drop into 5th will bog the engine down.
Last edited: