Originally Posted by MFactory
The final drive gear is exactly that, the final gear i.e it comes after all the other gears. It doesn't affect the other gears "mechanically", and is basically just a torque converter
A longer final drive (lower numerically) will be more durable than a shorter final drive in terms of the final drive breaking, but causes more radial loads due to the smaller diameter ring gear, and vice versa.
Let me reword that:
Longer Final Drive (e.g 3.7) - More durable, but increases radial loads
Shorter Final Drive (e.g 4.1) - Less durable, but decreases radial loads
Radial loads is what pushes the shafts apart, causing gear mesh misalignment and worn/cracked bearings, ultimately leading to increased tooth wear and/or breakages
Next question: Is there a way to compare acceleration rates between different gear ratios and final drives? ie 3.166 4.11fd 1st gear in an LGT vs 3.454 3.70fd 1st gear in an 07 WRX. They result in roughly the same top speed. Obviously, power curves effect the real world results, but I assume there's an equation to give a theoretical comparison?