The full details will be in the Game Mechanics section of the manual. Expect that part of the manual to be implemented in about a week unless I get bogged down in many more bug tickets in the next few days. (Currently, I am working on v2.0.0.3. Otherwise, I would be finishing up the chassis game mechanics portion of the manual at this moment.)

In short, the more gear options an engineer has, the more he can get maximize with gear ratios, increased torque in low gears, better fuel economy in higher gears, etc. As of v2.0, most of the gear's direct effect on specs/ratings are linear with the number of gears.

If you were to mod 100-gear gearboxes in the game, it would probably break the formulas and cause most ratings to be pegged at the maximum 100 value.

I considered modding in some modern CVT (for example williams 1993 F1 car with CVT) by saying it has some ludicrously high amount of gears... So I guess this is bad idea then.

I can just say CVT has the highest amount of gears as a manual gearbox allow, but not go beyond that.

CVT's are not implemented very well in the current iteration of GearCity. I'm hoping to fix it with the Feature Bounty system. As it stands now, they're semi-designed as the early 1930s style rather than modern CVT's. Many of these systems in the game predate the resurgence in popularity the CVT now enjoys.

So in short, best to wait until the feature bounty system funds some of the improvements i have planned for designs, as it should fix CVTs.