Vehicle & Wheel Speed

Applies to: IOcan · Standalone controller. What this means

The OEM gateway drivers must show the factory TCU a plausible set of wheel speeds — the TCU uses them for shift scheduling and plausibility checks — and they also provide vehicle speed outputs for the dash or other consumers. This page describes how the speed emulation works and what it needs; it is shared by the BMW OEM Gateway and the OEM PDK Gateway.

Required parameters

Parameter Meaning Unit
Diff ratio Final-drive ratio between the transmission output and the wheels :1, entered ×1000
Wheel diameter Rolling rolling diameter of the driven wheels mm

These two parameters convert between shaft speed, wheel speed and road speed.

Which ratio values are accepted

Any positive value. The device does not hold a list of permitted ratios and does not check yours against one — whatever you enter is what it uses.

  • Resolution is ×1000. A 3.64:1 final drive is entered as 3640, a 2.81:1 as 2810. Three decimal places are available; use them, because the error is proportional and applies at every road speed.
  • Only zero or negative is rejected. Either parameter at or below zero raises fault 0x0041 (parameter not set) against component 401 for the ratio or 402 for the wheel diameter. It is a warning, so the transmission still runs — but the tailshaft-to-wheel conversion is skipped and the gateway falls back to the vehicle speed input. If that is not assigned either, emulated wheel speed stays at zero.
  • There is no upper bound and no plausibility check on the value itself. A ratio entered as 364 instead of 3640 is accepted silently; the only thing that catches it is the cross-check below, and then only once the car moves.

Wheel diameter means rolling diameter, not tyre size. A 225/40R18 is not 18 inches — derive the rolling diameter from the full tyre size, or measure the rolling circumference and divide by π.

Set both parameters before anything else — shift scheduling is built on the speed picture they produce.

The transmission's coded ratio is a different value

There are two final-drive numbers in a swap, and they are not the same field:

Where Accepted values
The device parameter Diff ratio, above Anything positive. Enter your real ratio
The transmission's coded ratio Inside the transmission control unit One of fourteen fixed values — see below

The device parameter converts shaft speed to road speed. The coded ratio is what the TCU itself uses for its own shift scheduling.

Whether the coded ratio is restricted to a fixed set of values is not documented here, because we do not yet have a verified list. Treat the transmission's coding as its own exercise, using the tooling appropriate to that control unit.

Verify, do not assume

A ratio error is proportional and constant, so it produces a fixed offset between indicated and GPS speed at every speed.

The plausibility warning will not catch a small one. The gateway warns when the TCU's tailshaft speed and the vehicle-speed input disagree by more than about 10 %; an error of a few percent passes silently while still skewing shift scheduling. Treat the GPS cross-check in the setup checklist as mandatory rather than optional.

Speed sources and priority

The gateway builds the wheel-speed picture from the best source available:

  1. The TCU's own output-shaft (tailshaft) speed — once the TCU reports a valid, live signal that has shown real rotation, it is the preferred source: it is exactly what the transmission itself measures.
  2. The Vehicle speed input — used at power-up (before the TCU signal has proven itself) and whenever the TCU signal goes invalid or silent. Assigning it is optional but strongly recommended.
  3. Per-wheel speed inputs (front/rear, left/right) — when assigned, each one overrides the emulated value for that wheel, e.g. from an ABS module. Wheels without an input get the value derived from source 1 or 2.

Plausibility monitoring

While both the TCU signal and the vehicle speed input are alive, the gateway cross-checks them and raises warning faults (the transmission keeps running) when they disagree:

Warning Meaning Usual cause
Tailshaft signal stale The TCU stopped refreshing its speed signal CAN wiring / TCU dropout
Tailshaft stuck at zero The TCU reports standstill while the vehicle speed input shows motion TCU speed source failed; the gateway falls back to the vehicle speed input
Tailshaft/VSS implausible The two sources disagree by more than ~10 % Wrong Diff ratio, Wheel diameter, or vehicle-speed input scaling

Setup checklist

  1. Set Diff ratio and Wheel diameter.
  2. Assign the Vehicle speed input from the ECU or a sensor and verify it reads correctly at a known speed.
  3. Optionally assign the per-wheel inputs if the car has live ABS wheel speeds.
  4. Check the driver's Vehicle speed / Wheel RPM outputs against GPS — if they are off by a constant factor, revisit step 1.