BAD NEWS
Safe and Aggressive preset setups are fine except for one extremely important reason:
Tyre PSI is set so wrong that you will be sliding about and lacking in grip no matter how good you are as a driver.

Unfortunately to fix this you need to mess with your setup...
...and setups are scary

GOOD NEWS

I want to help you dip your toes into this world in an accessible way that assumes you have no idea what you are doing but know how to make car go vroom.

Just changing tyre psi will make your car faster in corners, more stable to drive and is easy for anyone to do with no experience as there are handy visual hints.

Why Tyre psi?
This element gives you a huge boost to laptimes and is easy to get in-game numerical or visual feedback from the game so you can adjust them correctly. Other factors of setup unfortunately require reliable lap consistency to see if you are making a good or bad change.


Disclaimer for people with 1000 hours in ACC and strong opinions:
I am well aware that this guide is limited and there are other methods. This guide is designed for simplicity and tangible benefits without needing to be a super consistent driver or have a physics degree to deal with suspension geometry and aero/ride height balancing.

Before we start, make sure you have the tyres overlay visible when you are driving.
It looks like this

Continuing on, this part is very important. Tyres make the majority of your track grip.
Getting the ideal pressure in the tyres is very important.


For all cars you should target between 26.0 psi - 27.0 psi


Here are the issues:
Tyres that are too hot or too cold will have less grip.
Tyre psi increases as the tyres warm up.
The temperature in the session may change over time.
On an clockwise track the left side tyres are likely to be much hotter than the right.
On an anti-clockwise track the right side tyres are likely to be much hotter than the left.
Some tracks really heat up the front or rear tyres.

The result:
We have to do the following method from scratch on every new track and weather combination.

Here is your tyre psi method:
Do 2 laps, do not spin or slide or it will invalidate the test.

Come to a gentle stop and return to garage.

Then go to the current setup and look for 'Psi hot'.



Target is 27.0 'Psi hot' on all tyres.
The reason for this is that 'psi hot' is the peak and we know anything between 26.0psi and 27.0psi is good.
If your 'Psi hot is under 27.0 then you should increase the psi setting on the tyre.
If your 'Psi hot is over 27.0 then you should decrease the psi setting on the tyre.
For example: If your 'Psi hot' is 25.2 I would increase that tyre's psi setting by at least +1.5 psi.

Do this for each tyre. Don't panic, this is all guesswork, no one gets this accurate after just 1 adjustment. Also the psi settings will probably not be symmetrical, this is normal.

Here are my guesses based on my psi hot settings


Do 2 more laps with the new settings.
Return to garage, go back to the tyres setup tab again and see what your new 'Psi hot' is.
Adjust your psi settings same as before. Remember the target 'Psi hot' is 27.0!

Repeat over and over until you are at (or really close to) 27.0 'Psi hot'.

Then do about 6 laps. See if the tyres stay between 26.0psi and 27.0psi on the overlay as you drive about. Longer stints will bring the brake temps into play as they will stabilise in temp after about 4-5 laps.

If the brakes seem to have started overheating and are hitting red you might have to open one of the brake ducts by +1 on the 'Aero' tab.
If you have to modify the brake ducts expect the tyres on that side of the car to increase or decrease in psi.

Then SAVE YOUR SETUP
Next time you return to this track with this car it is very likely that the temps will be different and you will have to make some small adjustments to the psi to suit the temps.
Because of this I cannot stress how useful it is to include the track temps in the setup name.

With your saved setup you can load it in future and have your brakes and tyres close-ish to a good temperature right from the start.

HOWEVER

It is extremely likely that your psi settings will be off by a bit and in some cases may go cold (blue) or hot (red) because the track has a higher or lower temperature than when you made the setup.

All you need to do is make small adjustments to compensate for this new ambient temperature.
Remember that the goal is 27.0 'psi hot' on every tyre. Use the method mentioned previously to get to that magic number.

After you have everything fixed:
SAVE YOUR SETUP AS A DIFFERENT NAME
Include the ambient temp and track temp as part of the setup name again. Ideally you will end up with a range of preset setups with different temps ready to go so that in future you can pick the setup that matches the current sessions the closest.

Good luck and enjoy your newfound grip!


I may expand this in future