EDITING TRACK FILES
A lot of the information on this page has been pulled from this in-depth breakdown by The Racing Line. Credit where credit is due. It's a wall of text from 1997, so this page will only have some of the crucial things you'll need, but there are other options you can read about in the breakdown.
First off, each track has a .txt file in its folder named the same as the folder itself. Atlanta will have a .txt file in its folder named Atlanta.txt. That's what you want to open. TRUCKS: Look for BLAPT and RELST lines.
CREATING YOUR OWN CALENDAR
The aforementioned Racing Line breakdown has detailed instructions.
PIT ROAD SPEED LIMIT
Change the speed limit on pit road if you want.
> PITSP 55
ADJUSTING A.I. PIT WINDOWS
Without wear mutlipliers in the game, it's still possible to have some short races with pit stops. The final two numbers on this line are the laps when the A.I. starts their pit window and when it ends, with maybe flexibility of a lap or two on each end. In this example, the A.I. will start pitting on lap 23 and finish their rounds by lap 26. This doesn't affect your own tire wear and fuel use. It's just a number that tells the A.I. when to pit, and you just have to treat it as a mandatory pit stop for yourself.
> SPDWY 0 23 26
PACE SPEED
55 miles per hour on the pace laps around Talladega is brutal, so you can change the speed here.
> PACEA 2 48080000 52000 -52000 0 55
QUALIFYING LAPS
The second number will dictate how many laps you can run.
> QUAL 2 2
QUALIFYING SPEEDS
If the A.I. is ridiculously fast in qualifying and you have no chance to compete with them, you can level the playing field here. This BLAP number is, roughly, what the middle-of-the-pack A.I. car will run in qualifying. If you run 30.450 in qualifying, you'll probably start middle of the field. I have noticed this number does rely on the difficulty percentage in some way. But once you try it out, you'll understand what's happening here. Change this number to 40450, and you'll win the pole by 10 seconds! My personal method is to run three qualifying laps, figure out what my average lap time is, then use that number as the BLAP in the track file. If I can improve, then I might start mid-teens or something. It's all up to you on how challenging you want it to be.
> BLAP 30450
RELS
This is a number that will adjust how fast the A.I. drives at the track. It's kind of a blind number, meaning I can't point to what it actually means, although it feels like it's a percentage of some kind. For example, if you open the Atlanta track file and change this number from 103 to 40, the A.I.'s lap times at 99% difficulty go from 30-second laps to 80-second laps. For your championship seasons where you are locked into a difficulty percentage, this line will come in handy. It will just take some experimentation based on your skill level. Judging by the drastic change seen in the 103-to-40 test, you won't have to change this number by too much.
> RELS 103
AIDMG
Adding this line to a track file lets you tell the game how frequently you want incidents, whether it's A.I. spin-outs or mechnical problems. The numbers are laps. The first number is always 0. In this example below, you can expect an incident every 10 laps.
> AIMDG 0 10
FIELD SIZE
Here is the excerpt from the actual ReadMe file that comes with the game.