This classification system is subject to change at any time.

STOCK
The idea of the Stock class is to provide a class where stock Miatas can be competitive.

Tires: Street tires of any width/size, but limited to a treadwear rating of 100+.
M1 1.6's allowed airbox, exhaust; M1 1.8 allowed exhaust
Alignment: any setting
Lowering springs
Any brake pad
Any modification to stock rotor
Any brake fluid
Brake ducting allowed
Brake dust shields can be removed
Brakelines
Any air filter
No appreciable weight reduction allowed
etc...

Modified (Formerly Stock 2)
Modified is the same as Stock, with the addition of race tires

All Stock-class mods, plus:
Tires: Any DOT tire, no slicks
Filter and airbox allowed for all cars
Exhaust: Allowed on all cars
Shock change allowed
Sway bar change allowed
No appreciable weight reduction allowed
etc...

Production (Formerly Mod 1)
Production is designed for those with mildly-modified cars.

All Modfied-class modificatons plus:
Tires: Any tire, slicks included.
Any mod short of power adders (Turbo, Super Charger, etc.). Engine work allowed. No appreciable weight reduction allowed


Unlimited
Unlimited is basically an "Open" class. Almost anything is allowed.

All Production-class modificatons plus:
Tires: Any tire, slicks included.
Alignment: any setting
Any suspension mods
Weight reduction
Turbo, Super Charger, whatever. No tube-frames, etc. These classes are for production-based cars.
etc...