Why do they overinflate new tires? Door says 35 lbs tires say 40 lbs I just checked them
Did you ask this question of the place that put too much air in the tires ? Without knowing who did it I doubt if anyone can say why they did it. But you have proved it always a good idea to check things like that.
Should use pressure listed on door. Pressure written on tires are just specs for maximum pressure allowed for not damaging tires. If you fill your tires to 35 when car is cold, then drive for awhile, tires get heated up and will probably read 40 psi when warm.
Just a guess. On installation sometimes they will overinflated to seat the tire on the rim. Then decrease the pressure to correct PSI. Perhaps they failed to reduce the pressure.
And as tcmichnorth said, if check tires are fully warmed up, pressure increases.