Tuning gear

When tuning, always you do it on the 4th gear of the vehicle.

Is it because the engine real output is same as when it run on the 4th gear?

And it means that, we don't want to tune the engine in other gears?

A chassis dyno allows tuning in different gears. For inertia type dynos, and taller gear ratio results in less effective torque at the wheel reducing the wheel spin problems. However on my Dynapack dyno, I often test in other gears depending on how well the dyno control is working. So low inertia race cars which have little torque (1 liter bike engines), often want to be tested in 2nd or 3rd gear.

The engine output is the same regardless of gear, the torque multiplier of the gear box is the only difference.

okay thank you, but,

How it be choose the correct gear for tune different cars?