OK, here is a start. I edited the ini/guide posted by Werner Winkels to add some more info. If anyone has additional info to add, post it and I will edit my post.
[GT3CarData]
Region= EU (Region= EU for PAL-Game) (Region= US for NTSC- Game)
BaseFile= File-Name
BaseName= Wagen-Name
0= Car Base Part 1
4= Car Base Part 2
8= Color Part 1
12= Color Part 2
16= Zulassung / Wagen-Art Part 1 ( 2=Race 1=Road )
20= -Zulassung / Wagen-Art Part 2
24= Brakes Part 1
28= Brakes Part 2
32= Brake Controllor Part 1
36= Brake Controllor Part 2
40= Chassis Part 1
44= Chassis Part 2
48= Engine Part 1
52= Engine Part 2
56= Drivetrain & VCD Part 1
60= Drivetrain & VCD Part 1
64= Transmission Part 1
68= Transmission Part 2
72= Suspension Part 1
76= Suspension Part 2
80= LSD Part 1
84= LSD Part 1
88= Front Tyres Part 1
92= Front Tyres Part 2
96= Rear Tyres Part 1
100= Raer Tyres Part 2
104= 17 ?? Part 1
108= 17 ?? Part 2
112= Weight Reduction Part 1
116= Weight Reduction Part 2
120= Body & DF Part 1
124= Body & DF Part 2
128= Polish Part 1
132= Polish Part 2
136= Balance Part 1
140= Balance Part 2
144= Displacement part 1
148= Displacement part 2
152= Computer Part 1
156= Computer Part
160= N\A Tune Part 1
164= N\A Tune Part 2
168= Turbo Part 1
172= Turbo Part 2
176= Flywheel Part 1
180= Flywheel Part 2
184= Clutch Part 1
188= Clutch Part 2 ...Remember that the value in the ini is 1 greater
192= Shaft Part 1 ....than the value in the game, so for instance to
196= Shaft Part 2 ....set the front DF to 250, you would set line
200= Muffler Part 1 ..292=-251. In cases where there is a decimal point
204= Muffler Part 2 ..such as gear ratios and spring rate, don't use
208= Intercooler Part 1 ...them in the ini, so a gear ratio of 1.635
212= Intercooler Part 2 ...would be entered as -1636
216= ASM Part 1
220= ASM Part 2
224= TCS Part 1
228= TCS Part 2
232= Wheels Part 1
236= Wheels Part 2 ___________________________________________
240= >R Gear ..Anything from this point down can be adjusted in the ini
244= >1 Gear ..but generally you would only change settings that you
248= >2 Gear ..can't change in the game, unless you wish to set things
252= >3 Gear ..outside the bounds of the game settings.
256= >4 Gear
260= >5 Gear ..For instance, you may wish to set the gear ratios here
264= >6 Gear ..if your using the car in a "normal car race" and so can
268= >7 Gear ..not buy an adjustable tranny. The same would apply for
272= >Final Drive ..suspension settings.
276= >Auto Gear
280= >VCD
284= Front Brake Adjust..These could be adjusted to a very high value
288= Rear Brake Adjust..like 100, which may help at high speed.
292= Front DF..DF settings really depend on the amount of HP and how
296= Rear DF...how an individual likes the car to handle. Generally
...............between -110 and -210 for moderate HP, and you might
...............try 0 or -251 for high HP or tighter handling.
300= Turbo 1
304= Turbo 2 ..As I understand it, Turbo 1-6 will adjust the percentage
308= Turbo 3 ..of turbo boost you will get in each gear. Not having
312= Turbo 4 ..messed with it myself, i can't comment any more than
316= Turbo 5 ..that.
320= Turbo 6
324= Front Camber
328= Rear Camger
332= Front Ride Height..The minimum setting here is -1(0 ride height)
336= Rear Ride Height ..The max is -251 (a monster rally car)
340= Front Toe
344= Rear Toe
348= Front Springs..A high setting here will obviously stiffen the ride
352= Rear Springs ..and can improve handling on the tarmac. A low
....................setting often works well on rally cars. I will
....................often use -2 on my rally cars, which is equal to
.................___.1lbs of spring rate, and combine it with high
.....................ride height.
356= Front Grip ( If the value to high, the screen begins to woble )
360= Rear Grip ( If the value to high, the screen begins to woble )
364= Front Bound
368= Front Bound
372= Front Rebound ..For bound and rebound, I often use below game
376= Front Rebound ..settings for my rally cars
380= Rear Bound
384= Rear Bound
388= Rear Rebound
392= Rear Rebound
396= Front Stabilizer
400= Rear Stabilizer
404= Front LSD Init
408= Rear LSD Init
412= Front LSD Accel
416= Rear LSD Accel
420= Front LSD Decel
424= Rear LSD Decel
428= Final Drive
432= TCS
436= ???
440= horse power modifier ( a Value from -1001 =HPx1 )
( A Value from -2001 = HPx2 ) ( A Value from -501 = HPx0.5 )
444= ASM
448= Driver A/MT
452= Driver ASM
456= Driver TCS
460= ???
464= Fittment 1
468= Fittment 2..Eliminating fittment values will allow any car to
472= Fittment 3..rally
476= Travel Meter ...a value of -1 sets milage to zero
480= Oil 1 ..a value of -1 means the engine is broken in
484= Oil 2 ..a value of 929924095 means oil is changed
488= shine of car ..a value of 0 or -0 is "super shine"
492= ???
496= ???
500= ???
504= ???