HUD Mode Explained — Turning Your Windscreen Into a Speed Display

A head-up display keeps information in the driver's line of sight so that reading it does not require looking away from the road. Aircraft have had them for decades; cars have had them since the late 1980s. A phone in HUD mode is a cheap approximation of the same idea, and understanding its optics is the difference between a useful tool and a distracting gimmick.

The mirroring

Lay a phone flat on the dashboard, screen up, and look at the windscreen. You see the screen reflected — but reversed left to right, like text in a bathroom mirror. HUD mode fixes this by flipping the display horizontally before it is drawn. The reflection then reads normally.

That is the whole trick. There is no projection, no special glass, no focus at infinity. It is a mirror image on a partially reflective surface.

What you gain and what you do not

You gain a readout roughly in your forward field of view, so your eyes travel less and refocus less than they would looking down at the instrument cluster.

You do not gain what a real automotive HUD provides. A factory unit projects an image that appears to float several metres ahead of the car, at a focal distance close to that of the road. Your eyes barely refocus. A phone reflection sits at the windscreen, roughly an arm's length away, so your eyes still have to change focus from the road to near. The benefit is real but smaller than the name suggests.

When it genuinely helps

At night. A windscreen reflects only a few percent of the light that hits it. In darkness a phone at moderate brightness produces a clear, readable reflection. In daylight, sky and road glare overwhelm it completely.

On a long, steady drive. The value of a HUD is proportional to how often you want the number. In stop-start city traffic you will look at the road anyway.

With a dark interface. A mostly black screen with a few bright digits reflects as bright digits. A bright screen reflects as a glowing rectangle that obscures the road — the opposite of what you want. This is one reason the app is dark throughout.

Setting it up

1. Select HUD from the mode selector at the top of the main screen.
2. Lay the phone flat on the dashboard, screen up, centred with your line of sight, as far forward as it will sit without sliding.
3. Use a matte anti-slip mat. It stops the phone moving under braking and kills the secondary reflection that a glossy dashboard produces.
4. Set brightness as low as remains readable. Too bright is worse than too dim — it obscures the road and ruins your night vision.
5. Check the reflection before you drive off, not while moving.

The legal part, stated plainly

Rules on screens and reflections in the driver's field of view vary widely and are enforced unevenly. Some jurisdictions prohibit any device that reflects into the windscreen; others regulate placement only; others say nothing specific. Using HUD mode lawfully is your responsibility, and no feature of any app changes that. If you are unsure, use the digital or analog face with the phone mounted conventionally — you lose a little convenience and no capability.

A last practical note: whatever the law says, anything loose on a dashboard becomes a projectile in a collision. A mat is the minimum; a proper mount is better.