How NASA TT Classing Works: The Weight-to-Power Math

Every TT1–TT6 entry in the paddock is built on one line of arithmetic:

Adjusted Wt/HP Ratio = (Competition Weight ÷ Average HP) + Sum of Modification Factors

Whoever understands that line best usually starts the season with a car classed exactly where it should be — and whoever doesn't donates lap time or collects a DQ. Here's each term, and where the traps are.

Competition weight: declare low, but survivable

Competition weight is car plus driver plus ballast — the minimum your car will weigh any time it competes. Impound checks it. Under your declared minimum: DQ. So the game is to declare the lowest number you can always make, accounting for fuel burn across a session. Weigh the car with you in it, minimal fuel, then declare with a small safety margin. Ballast is capped at 300 lb, solid and secured.

Weight also generates its own factor in TT1–TT3: under 2,200 lb is worth −0.3, while a 3,900+ lb car collects +0.6 or more. Heavy cars get ratio relief; flyweights pay for their agility.

Average horsepower: the dyno averaging rules

Your HP number comes from a certified three-run dyno session — Dynojet mandatory for 2WD. But the number that counts isn't peak; it's an average that depends on your engine's character (ST §6.3):

• Redline above 7,000 rpm: average of peak plus seven points at ±250 rpm intervals (divide by 8)
• 6,000–7,000 rpm: peak plus five points (divide by 6)
• Below 6,000 rpm, and all forced induction: peak plus three points (divide by 4)

The averaging rewards peaky NA engines and penalizes fat torque curves — a turbo car's average sits much closer to its peak. You may always use straight peak instead, or declare a higher number for headroom. That headroom matters because officials can demand a dyno at any time and GPS power monitoring is in use: dyno over your declared HP and it's a procedural violation even if your ratio still makes class. Full protocol in Do You Need a Dyno Sheet for NASA TT?.

Rounding runs in your favor: weight to the nearest pound, HP to the whole number with x.50 rounding down, and any ratio ending .995+ rounds up.

Modification factors: where classes are really decided

The base ratio gets adjusted by factors — tires are the big one. In TT1–TT3: 200TW street tires are worth +3.0, the fast 200TW group (RE71RS, Cup 2, A052 and friends) +2.0, unlisted DOT R-comps (Cup 2 R, Trofeo RS, Hoosier R7) +0.0, and slicks go negative. That's a three-point swing between a PS4S and a Cup 2 R — often the entire distance between TT3 and TT2. Tire choice isn't a setup decision in NASA TT; it's a classing decision.

Then the smaller factors stack: narrow tires give back up to +0.6, AWD costs −0.3, FWD gains +1.0, and transmissions get charged by class — in TT3 an OEM PDK/DCT is −0.5 while TT1/TT2 charge just −0.2 for any paddle box. Aero on production cars in TT1–TT3 is free within the universal limits (wing no more than 8 inches above roofline, front devices within 12 inches of bodywork). TT4–TT6 run their own tighter factor tables and tire eligibility — see the class-by-class breakdown.

Worked example, compressed

A 3,250 lb (with driver) car averaging 430 hp sits at 7.56 base — under TT2's 8.00 floor, so TT1 as it stands. Add Cup 2s (+2.0): 9.56, comfortably TT2. Same car on Cup 2 Rs (+0.0) is TT1 or bust. One tire decision, one full class. Run your own numbers in the TT Engineer tool, or see the full GT3 RS worked example including the active-aero complication.

The strategy layer

Declare HP high enough to survive any dyno day, weight low enough to be honest but competitive, and pick tires by which side of a class floor they land you on. Remember you can always run up a class — being 0.2 over a floor in the slower class beats being 0.2 under it and illegal. And once you're classed: no car changes after your first competition session without TT Director approval.

FAQ

What dyno do I have to use? Certified three-run session; Dynojet is mandatory for 2WD cars.

What happens if I dyno over my declared horsepower? Procedural violation regardless of whether your ratio still fits the class. First offense is a DQ; penalties escalate from there.

Do I have to hit my declared weight? You must never be under it when weighed in impound. Over is fine, slower, and common.

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