I usually perform this way:
- Reset to default
- Max speed to the minimum
- Final gear to the maximum
- Set again Max speed to the minimum
- Set the final gear to a meaningful value
- Adjust gears individually as needed.
The first four steps make gears close together, with a limited rpm drop. They also enable, by setting a tall final gear (low values), to have a long first gear, while other gears remain close together. These steps are necessary because for some reason programmers at PD thought it would be "cool" to limit the selectable gear range for each gear. Those limits however aren't realistic and do not make sense. The above described workaround mitigates this problem.
I usually set the first gear tall enough so that under full throttle the car will accelerate at its grip limit and will be able to do standing starts properly. On cars with low low-end torque often a compromise is needed. In general I aim to have a usable first gear. With default gearing, the first gear is most of the time useless as it's too short.
I set other gears so that the covered speed for each one is more or less constant or slightly decreasing as taller gears are engaged. This can be checked by examining the intersection points between the blue lines and the redline:
I do this process by eye, I don't do fancy calculations. This works pretty well.
The last gear is usually set up so that the car reaches its max speed at peak power rpm on the main straight at the Nuerburgring Nordschleife.
More than often after setting up the last gear for top speed, I quickly adjust intermediate gears in order to maintain the constant or slightly decreasing speed coverage for each gear (excluding the first gear) mentioned previously.
Once gears have been set up in this way, in order to not lose settings, further tweaks are usually performed by adjusting the final gear setting only (and/or the last gear). Using the max speed setting again messes up everything.
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In conclusion, the gear set up screen in GT5 is in general a bug-ridden mess ported over from GT4. To fully adjust gears, users have to use tricks and exploits. The gear graph is close to unusable, has no meaningful scale and shows strange and confusing overlaps. Also the lack (or better: removal) of a function to save custom settings has been a very amateurish move by PD. What the hell have they done?