lol you can't change gear ratios with a computer. You can change shift points probably in some cars.
you can change everything that is controlled electronically, such as speed limiter, sensor inputs, ignition timing, etc etc
but usually it requires a commecrical grade program such as autotap and a laptop with interface cable or a complete unit like hypertech ( i think that's what it's called ).
If you want to make your car faster, it won't work. You might gain a few HP, but nothing major. The purpose of those tweaks is to adjust the engine properly after doing hard modifications, such as cam swap, nitrious, or putting a set of new heads