Gran Turismo 4

This page details bugs of Gran Turismo 4.

Arcade Mode Car Switching Glitch#

If the player chooses a car in Arcade Mode and then switches to another car within a right timing, the color thumbnails will reflect those of the previously chosen car, while displaying the other car (as well its color names). This can be used to access special colors of certain cars in Arcade Mode, or to bypass dirt/snow or special car restrictions. Note that if the color thumbnail chosen exceeds what's available on the currently displayed car, the color name will mention BMW 120d '04 (occurring due to the car having the lowest filename, _120d_04); selecting it gives the first available color for the car. Additionally, some cars may need to be placed on the Favorites list for the glitch to be effective.

This glitch cannot be performed in the Japanese or Asian/Chinese versions of the game, and was reportedly fixed in the US Online Public Beta version.

Suzuka West Missing Description#

In the US version of the game, Suzuka Circuit West Course has a missing description when accessed from the World Circuits menu of GT Mode; if the track is highlighted there, the track's description text would be a placeholder text reading "CRSINFO|suzuka_west".

This was fixed on the US Online Public Beta version, using the European English text:

This is the west side of the Suzuka Circuit, with a unique series of hair-raising corners.

Driving Mission Completion Bug#

In the US version of the game, under certain circumstances, if the first Driving Mission (The Pass: Deep Forest) was not completed first, the mission may not be marked as complete no matter if the event has been finished or not, preventing the player from getting the DMC DeLorean S2 '04 (awarded for completing all The Pass Driving Missions) and 100% completion of the game.

Invisible Downforce Bug#

Equipping a rear wing from GT Auto will unlock downforce adjustment and increase the downforce ranges for both front and rear by 30. Unequipping the rear wing will revert the ability to adjust downforce, but the game will keep the car's downforce values intact. By equipping the wing, increasing the car's downforce, then unequipping the wing, the car will still have the benefits of high downforce while maintaining its stock appearance.

If the bug is performed on a car that already had downforce adjustment without the wing, it will still keep its high downforce as long as the "OK" button is not pressed on the downforce adjustment screen.

Gear Free Transformation Bug#

To generate a car's gear ratios, the game keeps track of the final drive at the time when the Auto slider was last set. For example, if the final drive is set to 5.500, the Auto slider is moved to 1, then the final drive is lowered to 3.000, the game stores 5.500 as an 'internal' final drive in order to re-build the car's ratios when the gear setup screen is re-entered. However, if the gear setup screen is exited after using the Auto slider without saving changes (either by pressing Triangle or the Cancel button), the game will still store the internal final drive without storing anything else. By re-entering the gear setup screen and adjusting the individual gear ratios, the game will use the internal final drive set previously, resulting in normally impossible gear ratios. This can create unusual scenarios, like a 3rd gear that's shorter than 2nd gear.

This bug can be used strategically by setting a final drive that you'd like to store, moving the Auto slider, exiting without saving and re-entering, adjusting individual ratios, saving your changes, then repeating this process until the entire gearbox is fully built. This lets you create extremely close gear ratios for minimal RPM drops between gears. This is especially useful for high-powered cars that want to be kept within a specific RPM range on long straights, as a long 1st gear can be utilized to reduce wheelspin and tyre wear out of corners while the rest of the gearbox is geared towards straight-line speed.

This bug would be fixed in the North American Online Public Beta version.

Grip Modifier Calculation Bug#

Several classic cars having grip modifier less than 900 results in them having performing better in certain conditions than intended due to an integer overflow in the calculation method: the grip modifier value is first subtracted by 1000 (-100 to +155), translated to an 8-bit signed integer (-128 through +127) and then turned into a percentage (-10.0% to +15.5%). In such cases, for instance, if a car has a grip modifier set to 899, the grip modifier would be at maximum (+15.5%) instead, making the car overpowered.

The following cars are affected, with cars that do return in later games having their front and rear grip modifiers set to 900 (the safest non-rollover minimum) there:

Additionally, in the retail Japanese version, the Chevrolet Corvette Z06 (C2) Race Car '63 has a grip modifier of 880 in the front and 930 in the rear, which resulted in the car being undrivable due to a front-heavy grip as a result of the aforementioned overflow. Later versions of the game correct this by adding 20 points each, to 900 front/950 rear, in addition to reduction in horsepower and a correction to torque graph.