When you make a dyno for your car it is not assigned by default. You have to assign it manually for it to take effect and be usable.

Instead of thinking about the tire pressure slider as a grip slider, Think of it as a "how much you turn" slider.



If you are turning too slowly, You are getting understeer. And if you are understeering you will have to slow down too much to get around corners. Which will cost you alot of time in races.

But if you turn too fast and oversteer, It might be uncontrollable and make you spin out.

                                       

The goal is to have the Tire Pressure as low as possible while it is still controllable. So you can turn as fast as possible and keep your speed around corners without losing control of your vehicle.
Some cars (FWD Cars mainly) can be easily controllable at -10 tire pressure. Test it out to see what you are comfortable with.

Put it to 10 for all cars. It makes the car grip better.

It does not effect acceleration or top speed. It only effects the cars controllability.

Center of Mass to the rear of RWD cars makes them turn more when going up curbs. Center of Mass to the rear also helps with making RWD cars more stable.

But Center of Mass too far to the rear can cause oversteer when going over curbs and can cause predictability problems.


In Progress

FWD cars like the center of mass to be in the front. +10 Center of Mass will work for most FWD cars.

Center of Mass to the front makes FWD cars much easier to handle. It gives more grip to the front wheels meaning less lift off oversteer but it can mean more understeer on acceleration.

AWD cars like the Center of Mass to be in the middle of the car. But 0 on the slider is not always the middle of the car.

The Kuruma has it's Center of Mass far to the rear by default, So moving the slider to +10 moves the Center of Mass closer to the middle for the Kuruma.
While the Sultan is already balanced, So it's perfectly fine with 0 Center of Mass.

The Drivetrain slider does mostly work how you would expect.
If you set the slider higher you will get a higher max speed, If you set it lower your max speed will decrease.
If you set the slider higher your acceleration will decrease, But if you set the slider lower your acceleration may also still decrease...

You can get the most acceleration on circuit tracks and URLs normally from -3 to +5 on the slider depending on the car.
Lower on the slider will give you better low gear acceleration for things like Street-X. But will give less high gear acceleration so you will actually go slower on long straights.

Moving the Brake Bias to the front will cause more understeer when braking. If your brakes are too powerful and are set too far forward, Your brakes will lock and you will slide forward with no ability to turn.
Moving the Brake Bias to the rear will cause more oversteer when braking. By setting it further back you can allow yourself to turn more while braking. But too much and you will spin.
                                                   Pro Brakes, -10 Bias.                                       

Stiffness is mostly simple.
If you see the body of your car sway and bounce when driving, The suspension might be too soft.
If your car drives over curbs like a plank, It might be too stiff.

Those types of big problems are pretty easy to fix, As there is a large margin of error.

Though, it also has some more subtle changes.
Changing the stiffness effects how the car reacts to bumps and curbs of course, And the goal is to set the stiffness so you can drive over curbs and bumps as smoothly and consistently as possible.
But how the car reacts can be completely different depending on car and dyno, Just continue testing for yourself to find the best for your own dyno or for a particular map/map type.

The main use of ride height is to help you get over curbs. Raising your car allows you to go over curbs and large bumps without the body of the car hitting the ground.

Lowering your car is usually a bad idea for street racing or anything with curbs. But slightly lower ride height can help with some maps such as bayview.
But it is recommended that you raise your ride height at almost all times. (Unless you have a already super high car such as a SUV or a Clover)
                                               
Example: In both gifs are the same dyno with the only difference being ride height. If the body of your car hits the ground you may be sent flying by curbs.

In Progress

Steering Angle effects how much the car turns at lower speeds. So if you set the slider higher and you let off the gas and start slowing down you will notice the car turns more.
But this is not always a good thing, With some cars you will oversteer when you try to accelerate at low speeds because you are turning too much. This can be helped by lowering Steering Angle.

But of course, Some cars understeer at low speeds and this can be helped by raising the Steering Angle.