Transmission parts increases your overall acceleration. Acceleration is an FR vehicle's best friend, since they have quite low acceleration. Your goal should be to increase your transmission aggressively to counteract slow FR acceleration.
Transmissions can be acquired from the Daily Transmission Event; you can only acquire up to Grade 11 from the Extreme category.
Parts are obtained from Story rewards, CP Orders, Gold Orders and Order Tickets. It is also possible to obtain these parts from the 24 Hour Part Races.