Citroen gearbox design is pretty old, conceived in late 50-s early 60-s.
In 50 years of existence, these boxes were coming in several different ratios, depending on the application, see attachment. Even you advertised several different ratios you had "in stock".
"Best" final ratio is determined not by the manufacturer, but by the particular Marque Ride Development Engineers on the basis of the desired dynamic performance of the vehicle.
Pitting is not caused by a difference in method of manufacturing, but due to... fatigue.
Tooth surface fatigue occurs when the load is applied on the tooth surface repeatedly, or when the force is applied on the tooth is larger than endurance limit of the material.
As the result of the surface fatigue, the material fails and falls off the tooth surface.
When the tooth surface strength is insufficient against the load, the following countermeasures may be applied:
** Change the material to a stronger material having more hardness and thicker hardened "skin". Take sufficient consideration on the depth of the hardening layer during heat treatment, to ensure sufficient hardening layer, and to keep the gear surface strength.
Especially, as for the carburized gear surface, in consideration that the core material is low-carbon soft tissue, which cannot bear the surface pressure from the hard surface of the gear surface and leads to surface crazing and hole erosion; therefore, it is necessary to pay special attention to the depth of the carburized layer, hence the insufficient carburized layer gives rise to the insufficient surface pressure strength which leads to hole erosion.
** Enlarge the gear size (my 4.286 CWP has tooth thickness increased by 2mm)
** Enlarge the face width
** Correct improper tooth contact caused by bad mounting. By increasing stiffness, tooth contact improves.
And the most important:
**Provide appropriate conditions for the lubricant; proper type, viscosity, and quantity.
Last, but not least, the new 4.286 ratio (comparing to OE 4.357) increases the vehicle speed in gears bringing up the speed increase over stock gear UN1-26 to +1%, whilst OE 4.375 keeps it at subdued -3% to -6%. !