When tuning cars via the Blackridge Rivals pre-race Tuning Screen it has no effect, tuning can only be applied via the garage.
What's causing this?
The car is already being loaded before the pre-race Tuning Screen, and at this point altering the car stats has no effect.
What can be done?
Sit tight, a fix is being worked on for the next update. In the meantime, unfortunately tuning can only be performed via the garage.