Initially, I made this mod for myself. Why did I make the mod for an almost 15 years old game? Because GTA 4 is one of my favorite games and I still play it sometimes. IMHO, GTA 4 has the best car physics in all GTA series, but I've always disliked boat-like suspension of cars.
First of all, I fixed the suspension of the cars in my mod. Then I noticed that most vehicles have an incorrect center of gravity, often too low. It took quite a long time to manually adjust the center of gravity of each vehicle. But the result was worth it. The vehicles began to feel quite realistic.
Then I started adjusting the tire grip, brakes, acceleration etc. As a result, I reworked all handling parameters of all cars and bikes based on real data.
I spent hundreds of hours on this mod, maybe even more than one thousand.
Once I gave my friends to play with my mod and they said that I should upload it somewhere.
So, here it is.

You can share this mod and publish it with credit to me as the author.
You can modify my mod for personal purposes, but you cannot publish a modified version.

More details about handling changes of cars and bikes:
- Fully reworked suspension of all vehicles: suspension strenght, travel, bounce, level, etc.
- Realistic acceleration throughout entire speed range.
- Realistic top speed.
- Brakes made about 10-15% stronger compared to real cars.
- Proper number of gears, more realistic gear ratio.
- Center of gravity manually corrected for all vehicles.
- Realistic weight distribution.
- Fully reworked tire grip for all cars.
- Realistic drag coefficient.
- Realistic weight for all vehicles.
- Some cars have changed from RWD to FWD.

Handling changes of helicopters:
- More realistic suspension.
- Realistic center of gravity.
- Realistic weight.

Compatibility:
The mod tested on GTA 4 version 1.0.7.0 and GTA 4 Complete Edition.
Compatibility with other versions shouldn't be problem either.

I recommend playing with a gamepad for the best experience.