Instructions
Buy a Full Racing Transmission from the Gran Turismo tune shop for the car you want to modify. Your car should be four-wheel drive or rear-wheel drive and have at least a 700 horsepower engine.
Choose the settings icon from the race menu. You might need to start a race and immediately push "Pause" to get to the menu. Select the wrench icon next to "Transmission" to modify your gear settings.
Slide the adjuster to left for higher speeds. Move it to the right for faster acceleration. Set each gear down three sets from the preceding gear.
Change the auto setting for a broader adjustment. Selecting a high number gears your transmission to reach high top speeds. Lower numbers gear your car to accelerate quickly.
Test your transmission's gearing in a practice race. If you have difficulty making turns, your car might be too quick for that particular track.
Return to the shop to make any adjustments before you race.