You need to equip all 6x Speed Cards with the (Green) Carbon cards. Doing so will give you a hefty bonus to Acceleration + AIR. (Great for an off-road build IMO)
The six Speed Card categories you need to fill are:
Block
Exhaust
ECU
Gearbox
Head
TurboTravel to a
Tune-Up-Shop and hit
when prompted. You'll be taken to a menu where you'll have three tabs at the top:
[BUY] - [EQUIP] - [TRADE-INS]
Hit
to go to the EQUIP tab and view which cards do not have a (Green) Carbon card equipped.
Once you know which cards you need, you can go the BUY tab and purchase the missing Speed Cards or go to the TRADE-INS tab and roll the RNG, there. I did this by selecting Carbon in the second column and pressing
to "Lock and Roll" It only took a few turns to complete my entire set.