The following is from Papyrus, who advise not changing the values marked with * (I would simply recommend backing up the original file before altering anything). Corey Rueckheim provides much more detailed information, including descriptions of the five TIRE lines which are definitely worth experimenting with.

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TNAME

Name of track in "Choose Track" menu

SNAME

Name of track in track standings

PIT*

Determines pit area and pit locations

SPDWY

Whether a superspeedway (0 = road course, 1 = superspeed-

way); determines which wings to use on the car, and the wing angle range

LENGTH

Length of the track; used only for display purposes

LAPS

Standard number of laps for a 100% race

FNAME

Full track name displayed on track info screen

CITYN

Track city and state/province displayed on track info screen

COUNT

Track country displayed on track info screen

PACEA*

Pace lap information

QUAL

Qualifying session type (0 = open session, 1 = lone average, 2 = lone best lap) and length (time in minutes or number of laps)

BLIMP*

Starting location of blimp

GFLAG*

Location of green flag trigger

TTYPE

Track type; determines whether to show lap speeds (ovals) or lap times (road/street courses)
   0 = short oval
   1 = mile oval
   2 = superspeedway
   3 = street course
   4 = road course
   5 = temporary street course
   6 = temporary road course
   7 = reserved oval
   8 = reserved road course

CARS

Number of cars to race against; random number in range

TEMP

Average temperature at time of day and year race is run, and range of variation; values determined from historical weather data; both values are in tenths of degrees Fahrenheit

WIND

Average wind direction and strength at time of day and year race is run, and range of variation; values determined from historical weather data; first two values are direction of wind and variation thereof, the second set are the speed in tenths of a mile per hour; Papyrus recommends final value not be changed

RAIN

Rain data; values based on historical weather data; first value is the number of days per month rain will occur, second value is reserved; not used in game

BLAP

Determines qualifying time of opponents; value in thousandths of a second [I believe this is average qual speed of AI]

RELS

Relative speed of opponents at this track, in %, for racing (not for qualifying)

 

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Note that the in-game opponent strength setting affects both qualifying and racing speeds at once; if you want to change either by itself, you need to edit the BLAP or RELS values in the TRACK.TXT. Also note, most TRACK.TXT values can be easily changed with Dane Johnson's Windows-based CRHQ utility.