The six Speed Card categories you need to fill with Americana brand cards are:
Block
Exhaust
ECU
Gearbox
Head
Turbo
Travel to a Tune-Up-Shop (this can’t be unlocked in your Home garage, even if you have a full set of cards) and use the Buy or Trade-In options to make sure all of the Equip slots have Americana brand cards.
A tip for using the Trade-In section is to select the brand you need in the second column and choose "Lock and Roll".