Solved

% at which tires need to be changed

Hi, I'm new here, and when doing my practice laps I saw that it says the % that the tires are used (I would prefer to see the mm of thread used but heh, this is a casual mobile phone game after all)

Do the tires work without a problem till 0%? Or is there a % number when the tires need to be changed ASAP? Thanks!

around 35% its best to switch tires

As Kevin said, 40% is kinda of ideal in most circumstances.
They start to lose performance below 50%, but usually low fuel helps you keep a good pace for a few more laps.

Also, the remaining tyre life is calculated by the percentage of wear you had in practice on what you have remaining on the start of the lap. So for
10% would be:
Starts at 100%
Start of lap 2: 90%
Lap 3: 81%
Lap 4: 73% (72.9)
...
End of lap 8: 43%, which is the time to pit

Casual mobile game as this isn't Assetto Corsa Competizione ????

I don't know how to quote in an edit, but thank you Juanito Jones, I didn't knew it was the 10% of the remaining thread

40 is the big drop off that you want to avoid if possible. There are certain circuits and situations that are exceptions though. I'll generally run supers to 35 minimum, softs to 40 and mediums to 45-50. I've never used hards in this game ??

You can still be competitive down to 40%, as being low on fuel on worn tyres is still quicker than being on new tyres but heavy on fuel. 
You will notice also that the wear stated in practice is more than it’ll be in the race. 
If it’s says 10% in practice it’ll probably average out at 8% in race -more you do, lower the wear per lap. 

I always use this for tyre use, for example practice lap says 8%

8x0,75 (coefficient I always use) =6 (6% real wear). As you don’t want to go below 40% wear, you can use 60% so 60 / 6 = 10 laps max
Works fine for me

yes that’s what I do, from practice wear I’ll go to 80% (so if it’s at 8% - 10laps) and find that in race it ends up 60%

Yeah generally down to 35% for supersofts and 40% for softs are the optimum ranges. The wear does decrease during the stint so as the guys said above, practise percentages are really only used as a reference/guide. The actual % is always going to be a little less than the practise figure provided. A good way to calculate it is to get the average % wear throughout a stint. You can find all the figures on the race data page. So for example, you do a stint on soft tyres for 19 laps. Your tyres went down to 40% after 19 laps, so your total wear on that stint is 60%. Average wear of tyres ~ 60/19 = 3.158%  ...  In this example it's quite possible you would have been given a practise percentage of 5% which if remained consistent throughout the stint would only allow for 12 laps ~ Total wear divided by wear per lap 60%/5. But because it drops off throughout the stint and I now have my average or exact % wear, I can use the 3.158% as a good guide to exactly how many laps I want to do in future races on this track. Just remember, the race data in the game is only stored for 60 days after which it's no longer available. Keeping records can be very useful ?