NASA TT Classes Explained: TT1–TT6, TTGT, TTU, TTEV

Ten classes, three different logics. Get the logic and the whole rulebook stops feeling like tax code.

NASA TT classes work three ways: the unlimited classes (TTU, TTGT) have no math at all; the ratio classes (TT1–TT6) are set by adjusted weight-to-power; and the format classes (TTEV, TTT, TT Street) sit alongside on their own criteria. Here's each one, and who it's actually for.

The unlimited classes: TTU and TTGT

TTU is anything. No classification form, no dyno, no ratio. Sports racers, prototypes, unlimited aero, active aero — all legal. TTGT runs the same "no form, no dyno" freedom under GT rules but bans sports racers and prototypes: it's the unlimited class for cars that still look like cars.

These two classes matter to Porsche GT owners for a non-obvious reason: factory active aero is illegal in TT1–TT6. A 992 GT3 RS with functioning DRS is a TTGT/TTU car unless you disable and fix the system. If your car's headline feature is a moving wing, your class decision starts here — full analysis in the GT3 TT class guide.

The ratio classes: TT1–TT6

Each class has a floor your adjusted weight-to-power ratio must meet or exceed:

Class	Adjusted ratio floor	Character
TT1	6.00:1	500+ hp production cars, Cup cars
TT2	8.00:1	GT3s, big-power builds
TT3	10.00:1	Caymans, M3s, well-built momentum-plus cars
TT4	12.00:1	Production street cars only, tire and aero restricted
TT5	14.50:1	Factory ≤264 hp (≤275 FWD), under 5.1L
TT6	19.00:1	Factory ≤168 hp (≤174 FWD), under 2.45L, NA only

The "adjusted" part is where classing is won and lost: competition weight divided by average dyno horsepower, then modification factors added or subtracted for tires, weight bracket, transmission, drivetrain, and chassis. Sticky tires push you toward a faster class; a heavy car buys you factor room. The full math with worked examples is in How NASA TT Classing Actually Works — or skip the homework and run the TT Engineer tool.

Two structural notes. TT4 is production street cars only, with the tightest tire and aero restrictions of the ratio classes. TT5 and TT6 add factory-horsepower and displacement eligibility gates before the ratio math even starts — TT6 also bans forced induction and, in a very specific rulebook grudge, the 2006+ MX-5.

You may always run a faster class than your ratio dictates. Sandbagging down a class is what gets you DQ'd; declaring up is always legal.

The format classes: TTEV, TTT, TT Street

TTEV is the electric class: near-stock EVs on 180TW+ tires (or R888R/RA-1, max 305mm), no power or battery modifications, with tech per HPDE rules. TTT (Time Trial Target) is new-generation scoring — a consistency competition every entrant is automatically part of, which means your first weekend scores you in two competitions before you've learned the track. TT Street (new for 2026, detailed in the rules changes article) overlays your normal class: 200+ treadwear tires plus current street registration makes you eligible for separate street-car awards.

Where should you start?

If your car is bone stock and street driven: run your ratio class on TT Street rules and be competitive without buying anything. If you're on slicks with aero: expect TT1–TT3 depending on power, or TTGT if you'd rather skip the paperwork entirely. If you're between classes, remember the floor logic — you can't be under it, but there's no penalty for being comfortably above it. Guidance by car type in What's a Good First TT Class?, and the full on-ramp is in the complete getting-started guide.

FAQ

Can I run a faster class than my ratio dictates? Yes, always. The floors are minimums. You can never run a slower class than your adjusted ratio allows.

Which classes skip the dyno and classification form? TTU, TTGT, and TTEV. Everything TT1–TT6 files the online classification form, printed copy to the Regional TT Director.

What is TT Street? A 2026 overlay competition: run your normal class, and if you're on 200+ treadwear tires with valid street registration, you also score in the Street standings.