Instructions
Load the Forza Motorsport game and press the action button (A) on your Xbox 360 controller to access the Arcade menu. Press the right arrow on your controller to select "Free Run." Select the "D Class" of cars and use the right arrow to scroll through the menu to the 1987 Buick GNX. Press "A" to select "GNX" then press "A" again to select a track and racing options. Press the down arrow to select "Tune Car."
Press the right trigger button on your controller to scroll across the tuning options available at the top of your screen then press "A" to select an option to tune. Options include tires, transmission, suspension, aero, brakes and differential. Press the right arrow on your controller in the Tire section to increase your tire pressure. Press the left arrow to decrease tire pressure. Press the "Y" button within any option for context-sensitive help regarding the impact of tuning adjustments you make for each performance parameter listed. Repeat this process for each tuning parameter that you want to adjust then press "A" when you have completed your tuning adjustments.
Press "A" on your controller to select the "Start Race" option. Press the up arrow to access the Telemetry screen to monitor your car's performance during the race. Press the right arrow button within the Telemetry window to monitor acceleration, speed, braking, tire pressure, tire temperature, suspension and the other chassis tuning parameters listed. Continue to race and tweak your tuning setups based upon the performance of the GNX in the race. You can name and save your tuning setup when you select the option to "Finish Race."