Check out drivetrain.ini
[AUTO_SHIFTER]
UP=7400
DOWN=3800
SLIP_THRESHOLD=0.95
GAS_CUTOFF_TIME=0.190
As you can see the UP shifting is over the limit of the car (engine.ini)
[ENGINE_DATA]
ALTITUDE_SENSITIVITY=0.1 ; sensitivity to altitude
INERTIA=0.115 ; engine inertia
LIMITER=6600 ; engine rev limiter. 0 no limiter
LIMITER_HZ=40
MINIMUM=2000
[DAMAGE]
TURBO_BOOST_THRESHOLD=1.2 ; level of TOTAL boost before the engine starts to take damage
TURBO_DAMAGE_K=30 ; amount of damage per second per (boost - threshold)
RPM_THRESHOLD=7000 ; RPM at which the engine starts to take damage
RPM_DAMAGE_K=0.5
You have to change that UP shifting RPM, looking for example to ai.ini
[GEARS]
UP=6350
DOWN=3800
Try now
I've used 6400.