For you transmission gear heads

[Mach 5]

Say you're coming up to a corner and you're transmission needs to shift to 3rd gear before the apex. Which gear do you need to adjust, 2nd or 3rd?

Thank you in advance.

 

[Bowtie-muscle]

I would make second shorter, shifting into 3rd sooner if I understand you correctly. Assuming we are talking about accel and not decel. This will actually help handling through the apex as well.
If we are talking decel, I would make 4th just a touch longer. Causing a downshift just after the apex for better exit and launch down the follwing straight.
Just my approach, others may disagree.

 

[Mach 5]

I'm refering to the car still accel. before the braking zone. The car's transmission is wanting me to shift into the next gear. So then I do need the adjust 2nd or 3rd?

 

[Bowtie-muscle]

If it's just before a braking zone, make the current gear longer so you don't have to shift.

 

[Otaliema]

If it's a track optimized transmission than as @Bowtie-muscle said make that gear longer.
If it's a general transmission for lots of tracks, if available just lengthen the final gear, it will slow you down a bit but that's better than tweaking each gear for every race.