---
type: report
date: 2026-10-15
---
# Z27AG CAN: third pass, the drive back

Sessions `000011-20261015T212904` and `000013-20261015T214544` (the drive back, about 13 minutes,
no marks) together with pass 2's `000008` and `000009`: 20 km of driving with GNSS and the calibrated
IMU, no OBD-II. Nothing was marked, so this pass uses what any drive has: corners and distance.

## Confirmed

| Signal | Where | Evidence | Registered as |
|---|---|---|---|
| Wheel speeds | `235` four words, little-endian (pass 2): bytes 0–1 **front right**, 2–3 **front left**, 4–5 **rear right**, 6–7 **rear left** | from corners, below | `wheel_speed_fr_kph`, `_fl_`, `_rr_`, `_rl_` |
| Odometer | `570` bytes 4–6, little-endian, **1 km** | steps once per kilometre: 12 steps over 11.9 km (`000009`), 3 over 3.0 km (`000011`); 167966 on the 13th, 167988 at the end of the drive back; valid with the engine off too | `odometer_km` (the layout format gained 24-bit fields) |
| Distance counter | `14D` bytes 6–7, little-endian, **about 0.40 m** per count | tracks distance from the wheel speeds with correlation 1.0000 in both drives (103.2 and 103.6 m per step of byte 7); wraps every ~26 km | not registered: the odometer covers whole kilometres; this is there if a trip meter needs finer steps |

### Which word is which wheel

In a corner the outer wheels run faster than the inner ones, by about half the track width times the
curvature; and the fronts run on a wider arc than the rears, by about half the wheelbase squared times
the curvature squared. Fitting each word's share of the four (its speed ÷ their mean) against the
curvature from the steering angle (`steering_angle_deg` + 2.7° for the straight-ahead offset, ÷ 16.3,
over a 2.5 m wheelbase), 74 476 frames above 15 km/h:

| Word | Linear term (track) | Square term (wheelbase) | So |
|---|---|---|---|
| 0 | +0.72 m | +1.4 m² | right, front |
| 1 | −0.72 m | +1.4 m² | left, front |
| 2 | +0.75 m | −0.9 m² | right, rear |
| 3 | −0.75 m | −1.9 m² | left, rear |

Curvature is positive turning left, so a positive linear term is a wheel on the right (outside in a left
turn). The size, ±0.72–0.75 m, is half the Colt's track (1.45–1.46 m). The square terms split the words
into a pair above the mean and a pair below it, ±1.5 m² against an expected ±1.6 m² (2.5² ÷ 4). Medians
by curvature agree: in left turns tighter than 20 m (`κ > 0.05`) the words read 1.068, 0.956, 1.044,
0.932 of their mean.

Straight ahead the fronts read 0.6 % more counts than the rears, matching pass 2's per-word scales
(0.0578 against 0.0581 km/h per count). Driven wheels or tyre sizes, either is ordinary; each pair is
registered with its own scale, so all four read GNSS speed when cruising.

Jack stands (checklist 23–26) would still confirm this, but are no longer needed to find it.

## Candidates

- **`570` bytes 2 and 3** move together (byte 3 ≈ byte 2 + 48) and fall slowly: byte 2 read 87 on the
  13th and 76 at the end of the drive back, 22 km later. Fuel level fits the fall, but byte 2 also rose
  by about 3 while parked between drives (78 → 81), which fuel would not do on level ground. It may
  be outside temperature instead, falling through the evening. Both read `7F` with the engine off. A
  refuel settles it: fuel jumps, temperature does not. Not registered.

## Notes on the recording

- `000012` is a 25-second parked session in between; `000013` ends with the PEAK unplugged.
- The recorder kept IMU samples as JSON lines (34 MB for `000009`). From this pass on it packs them
  (`imu.bin`, svc-core `imu/packed.rs`): the same samples are 8.5 MB, and all of `000009`'s samples fit
  the packed form exactly.