Next year's development

oke i will try it. Maybe it's the solutions.

My season just ended and my new car parts are on average 76%, it takes 135 designers to update 3 parts next day and thoes 3 parts upgrade total of 10%, my season is about 40 days (mon wed fri races) so thats at best 57% average for next years car, and thats not taking into account that this years car upgrades 1% even when design emphasis is all on next seasons car, so i expect this will make progress even slower over the course of the season, so next seasons car could be as bad as 50% average.
So you cant realy always have a great car, the sweet spot looks to be around 60-65%, or you can have 1 alternate between a great car and an average car each season
This is based on a 1 car team and 160 staff level and 3 races a week.

after another day has passed some of the parts even with max designers assigned wont be ready for 2 days, so my next years car may be as low as 30%

Don't forget that at level 8 you can buy Wind Tunnel ans Test Rig Facilities that will accelerate development, without those we can't ask for a competitive car every season.