>>16777975 (OP)No. GTA IV does not use one identical gear ratio for every land vehicle. Each vehicle has its own handling data, including transmission-related parameters, and they can be different.
In GTA IV, vehicle behavior is controlled mainly through the handling files (especially handling.dat). A vehicle entry contains parameters such as:
fInitialDriveMaxFlatVel – theoretical top speed
fInitialDriveForce – acceleration/engine force
fClutchChangeRateScaleUpShift / DownShift – how quickly it changes gears
nInitialDriveGears – number of gears
fDriveInertia – how the engine responds to throttle changes
However, GTA IV is not like a detailed racing simulator where you can directly enter individual gear ratios (1st gear, 2nd gear, 3rd gear, etc.) for every vehicle. The game uses a simplified transmission model. The "gear ratios" are mostly derived internally from the vehicle's parameters.
You can make different vehicles behave as if they have different gear ratios by editing their handling values. For example:
Increase fInitialDriveForce → stronger acceleration in every gear
Increase fInitialDriveMaxFlatVel → higher effective gearing/top speed
Change nInitialDriveGears → more or fewer gears
Change shift rate values → faster/slower gear changes
But you cannot natively make a car have something like:
Vehicle A:
1st: 3.5
2nd: 2.1
3rd: 1.4
4th: 1.0
Vehicle B:
1st: 2.8
2nd: 1.8
3rd: 1.2
4th: 0.9
without deeper modifications (such as engine code changes, plugins, or custom physics systems).
Mods for GTA IV can go further. Some trainers or ASI plugins can alter vehicle physics dynamically, but the stock game only gives you the simplified handling parameters.