Repeated information across screens needs a designated source of truth

What it is

Speed can appear on the cluster, the HUD, and the navigation map at once; remaining range can appear on the cluster and on an energy page. Duplication can shorten a glance, but only after a source of truth is named: calibration, arbitration, and “which number counts” all point there. Everything else is a copy. Without a name, people drive by whichever surface they saw last, or whichever looks more finished—and that surface may be rounded by navigation, delayed, or fed by another sensor. This entry is authority once screens already agree. Agreement is “do not fight.” Authority is “if they briefly differ, people know whom to trust.”

Why it happens

Copies get rounded for layout, delayed, or converted to another unit. A HUD may show integer speed for readability, a map may estimate from its own fix, the cluster may read the speed sensor. People have no model of three screens doing display transforms; a visible gap is read as a fault or as two truths to choose from. Naming an authority is naming an arbitration rule: a copy must be explainable from the authority, and when the gap cannot be explained the copy yields or is marked non-metering. Driving-critical quantities usually belong on the cluster, the place already occupied by regulation and schema. Navigation detail belongs on the stack. A HUD should almost always be a copy—it trades precision and completeness to sit in the forward view.

Studying it

Let the same quantity appear on two screens at different precision or delay, with or without an explicit “cluster is authoritative,” and vary the size of the gap.

Independent variables: whether authority is marked, which screen holds it, delay and rounding of the copy, whether the gap is visually marked. Dependent variables: which surface people report as counting, which number they regulate to, extra glances when a gap cannot be arbitrated.

Do not treat “last surface looked at” as a successful authority design—that is proximity, not a rule. Ask “which number would a speed camera use,” not “which do you prefer.”

Where it stops holding

A quantity that lives on one screen has no authority problem. Entertainment metadata (track title) may be authoritative on the stack and need not enter the cluster. Legal metering of speed has accuracy requirements in many markets; authority cannot be assigned to a navigation map. If a concept cabin deletes the cluster, authority must be moved onto the remaining surface in so many words, and every copy must acknowledge the move; the HUD does not inherit by default. A desktop “main display” governs windows and the menu bar, not a quantity with physical consequences.

Applying it

• Name an authority screen for speed, gear, range, and control authority (usually the cluster). Show copies with fewer digits or an explicit “about,” so two surfaces do not both look like meters.
• Treat HUD speed and limit as a projection of the cluster, not a second localization-based estimate.
• When copy and authority differ by more than display rounding can explain, drop the quantity on the copy or show “see cluster”; do not keep two polished numbers side by side.
• Verify by giving HUD and cluster speeds a gap rounding cannot explain, and asking which number would count in a speeding citation. “I don’t know” or “the HUD” means authority was never named or never believed.

Related

• Within the group: K6.11.1 The cluster shows driving-critical real-time information, not optional functions · K6.11.2 The center stack carries navigation, media, and other secondary work · K6.11.3 Secondary displays and HUDs must agree with cluster and stack, not contradict them
• Adjacent: K6.05 Head-up Display · K2.02 Multiple Displays
• Search terms: source of truth · instrument cluster · HUD speed · display authority