Adjusted Wt/HP Ratio = (Competition Weight ÷ Avg HP) + Σ Modification Factors. Competition weight = car + driver + ballast, minimum any time it competes. Avg HP comes from a certified 3-run Dynojet report: for >7000 rpm engines, Avg = (Peak + 7 highest points at ±250rpm steps)/8; 6000–7000 rpm: (Peak + 5)/6; <6000 rpm and all forced induction: (Peak + 3)/4. You may always use Peak HP instead (or declare a higher number for dyno-variance headroom).
Class floors (ratio must be ≥): TT1 6.00 · TT2 8.00 · TT3 10.00 · TT4 12.00 · TT5 14.50 · TT6 19.00. Below 6.00 → TTU/TTGT. You may always run a faster class than your ratio dictates. TTU/TTGT require no classification form or dyno.
Key modification factors (TT1–TT3)
Weight: <3000 −0.1 / <2600 −0.2 / <2200 −0.3 ·· >3300 +0.1 / >3400 +0.2 / >3500 +0.3 / >3600 +0.4 / >3750 +0.5 / >3900 +0.6 / >4050 +0.7
Tires: 200TW street group (PS4S etc.) +3.0 · Cup 2/A052/RE71RS group +2.0 · RE71RZ/SC3R/P-01R +1.0 · unlisted DOT R-comps (Trofeo RS, R7, A055…) +0.0 · non-DOT slicks −0.5 (TT1/2), −0.7 (TT3)
Tire size: NASA SW ≤266mm +0.6 · 267–282mm +0.3 · printed >315 −0.3 (TT2/TT3, ≤3750 lb)
Transmission: TT1/TT2 sequential/paddle/DCT −0.2 · TT3 OEM DCT/PDK or 6+A/T −0.5, SMG −0.3, other seq −1.0
Drivetrain: AWD −0.3 · FWD +1.0 · Chassis: non-production −0.4 · sports racer TT1 −2.5 / TT2 −3.4 · TT3 BTM aero +0.4
Aero in TT1–TT3 production cars is otherwise FREE (wing ≤8" over roof, splitter ≤12" protrusion, ≤6" sides, no active aero).
TT4 differences
Production cars only, DOT non-autocross tires only, hard width caps (≤282mm SW >3000 lb / ≤266mm ≤3000 lb). Tire factors +2.0/+1.4(+1.0 from 3/30/26)/+0.5/0.0/−1.0(R8). Aero restricted; assessed items: 4–6" splitter −0.3, widebody/fender vents −0.3, canards −0.3, non-OEM diffuser −0.3, BTM aero +0.4. DCT/PDK/6+AT −0.5. AWD −0.5, FWD +1.0, solid axle +0.3.
TT5/TT6 eligibility & factors
TT5: factory ≤264 hp (RWD/AWD) or ≤275 (FWD), <5100cc. TT6: ≤168/174 hp, <2449cc, no forced induction, no 2006+ MX-5. DOT tires only with width caps by weight. Factors include suspension design/mods, shocks, brakes, engine location (rear/rear-mid −0.4), wing/splitter (TT6 −1.0 each), DCT −0.3, AWD −0.5.
Compliance reality
Dyno on demand + weight in impound; under min weight = DQ. Over declared HP = procedural violation even if ratio is legal. Declare HP high and weigh heavy for headroom. Active aero (including OEM — GT3 RS DRS) is illegal in all ST/TT classes; run TTU/TTGT to keep it.
Estimation constants used by this tool (assumptions, editable in source)
Avg HP ≈ 0.97 × Peak (NA >7000 rpm), 0.985 × Peak (FI/low-rev) · Driver default 180 lb · Lap deltas per ~2.2mi: slicks→DOT-R ≈ +1.2s · →+1.0-group ≈ +2.2s · →+2.0-group ≈ +2.8s · →200TW ≈ +4.5s · ballast ≈ +0.15s/100 lb · power ≈ +0.15s/10 whp (at ~470 whp; scales with %).
