Short answer
The stock speedometer is doing its job
FH6’s regular speed display follows vehicle speed: how quickly the car is travelling across the ground. That is the useful number for braking points, speed limits, and race timing. It stays readable when the tires lose grip because it is not a wheel-speed display.
Wheelspin lives in the gap between ground speed and wheel-indicated speed. If the driven tires rotate fast enough to imply 90 mph while the car is moving at 55 mph, the normal HUD still reports roughly 55 mph. Under heavy braking, a locked tire can imply nearly 0 mph while the car is still sliding forward. Ground speed hides both events by design.
Two honest answers
Ground speed and wheel speed answer different questions
| Measurement | What it answers | What happens when grip breaks |
|---|---|---|
| FH6 vehicle speed | How fast is the car moving across the map? | Continues to follow the car, even if a driven tire spins or locks. |
| Wheel-indicated speed | What road speed does driven-wheel rotation imply? | Rises above ground speed under wheelspin and falls below it during lockup. |
Inside the calculation
Wisp has to learn the missing piece first
FH6 Data Out supplies ground speed, four wheel angular velocities in radians per second, drivetrain type, tire-slip signals, steering, acceleration, braking, and suspension travel. It does not supply tire circumference. Without that last value, angular velocity cannot be converted into the linear speed of the tread.
Wisp solves for an effective rolling radius during clean telemetry frames. For a driven axle, each candidate is approximately r = v / ω: vehicle ground speed divided by the mean angular velocity of the two driven wheels. Samples are rejected while the car is braking, cornering hard, unloading the driven axle, spinning a tire, or producing wheel speeds that disagree by more than ten percent.
One plausible frame is not enough. The estimator keeps a window of candidates and waits for a dominant cluster: at least twelve accepted samples, at least sixty percent consensus, and no more than 1.5 percent radius spread. AWD cars are calibrated with separate front and rear radii, so a staggered setup is not flattened into one convenient but wrong tire size. A possible tire-size change faces a stricter replacement pass with 96 samples, ninety percent consensus, and 0.3 percent spread.
Every telemetry frame
The useful signal appears when the two speeds separate
Once the radius is trusted, the live calculation is mechanical: wheel-surface speed is the axle’s mean angular velocity multiplied by its learned radius. FWD uses the front axle, RWD uses the rear, and AWD averages the independently calibrated front and rear axle speeds. Wisp then converts the result from metres per second using exact unit constants rather than rounding through an intermediate display value. Optional smoothing is a time-based exponential filter applied after the physical calculation, and it is hard-limited to 1.5 mph from the current raw result so presentation can never overpower the telemetry.
Ground speed and wheel-surface speed can now be compared on the same scale. A positive difference means the driven tires are travelling faster than the car: wheelspin. A negative difference means the tire surface is lagging behind the car, which is what wheel lockup looks like. That signed gap is the real wheelspin signal, derived from measured motion rather than inferred from engine RPM or gear. Wisp’s traction-change logic also evaluates the normalized error |vwheel − vground| / |vground| alongside FH6’s driven-tire slip ratios, so one noisy channel does not decide the state by itself.
The stock speedometer is inferior for this job because it collapses the problem to one number: vehicle velocity. A car moving at 55 mph reads about 55 whether its driven tires imply 55 mph, 90 mph during a burnout, or nearly zero during a locked-wheel slide. That stable number is fine for road speed, but it has no observability into what the tires are doing. Wisp preserves both measurements, which is why lost traction becomes visible instead of disappearing behind a calm speedometer.