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 continues to show ground speed when the tires lose grip.
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.
Comparing the readings
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. Wisp learns the tires' effective rolling radius to convert angular velocity into wheel-surface speed.
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.
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 to account for staggered tire sizes. 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.
Ground speed and wheel-surface speed can now be compared on the same scale. A positive difference means the driven tire surface is travelling faster than the car, which can indicate wheelspin. A negative difference can indicate wheel lockup. Drifting and airborne motion can also separate the readings, so the gap needs context. It comes from measured motion rather than engine RPM or gear. Wisp’s traction-change logic also evaluates the normalized error |vwheel − vground| / |vground| alongside FH6’s driven-tire slip ratios to combine both sources of traction data.
Wisp preserves both measurements so you can see how the tires respond as traction changes.