To use your phone as a head-up display, open a speedometer app with a HUD mode before you drive, switch the mode on, and lay the phone flat on the dashboard, screen up, near the base of the windshield. The app shows your speed mirrored on a black screen, so the glass reflects it the right way round and your speed appears near your line of sight. After dark it works surprisingly well. In daylight it mostly washes out, it shows speed only, and it’s only as safe as the setup you do before you pull away.
How does a phone HUD work? #
Any glass reflects a little light. At night, with the cabin dark and the road ahead darker, a bright object on the dashboard shows up clearly in the windshield. The catch is that a reflection is reversed left to right, so a normal screen reads backwards in the glass. HUD mode flips the image first.
A usable phone HUD needs:
- A mirrored readout, so the reflection isn’t backwards.
- A pure black background. Black pixels give off almost no light, so only the digits reflect. A grey or dark-blue background would lay a faint glowing rectangle over the road.
- Big digits. The reflection is much fainter than the screen, and small text simply disappears.
Factory HUDs use optics to make the image appear to float out over the hood. A phone’s reflection sits at the glass, so your eyes still refocus a little when you glance at it. It’s a shorter glance than looking down at the dash, not a replacement for a built-in HUD.
What does Speedometer GPS’s HUD mode show? #
In Speedometer GPS, HUD mode is a full-screen readout of your speed and its unit, in large green digits on pure black. While it’s on:
- the digits are mirrored by default, and a flip toggle switches to a normal view if you’d rather use the phone in a mount
- the screen runs at full brightness and stays awake until you tap Exit HUD
- landscape is allowed, for bigger digits
- the digits turn red with an OVER LIMIT label when you pass your speed alarm limit
HUD mode shows speed only. Distance, trip time and your other trip stats stay on the main screen, so check those after you park.
How to set up a phone HUD, step by step #
Do all of this parked. Once you’re moving, the phone is a display, not something you touch.
- Open the app and wait for a fix. The GPS pill under the main readout should say GOOD or EXCELLENT (an accuracy radius of 10 m or less).
- Choose km/h or mph with the unit button, to match the signs.
- Set the speed limit alarm in Settings if you want one. It’s off by default.
- Tap Start if you want the drive recorded as a trip.
- Plug the phone in. Full brightness plus a GPS fix every second is a heavy load on a battery.
- Tap HUD and lay the phone flat, screen up, with its top toward the windshield, in front of the driver’s seat. If the number looks upside down in the glass, turn the phone around.
- From your normal seating position, slide the phone until the digits sit low in the glass, just below your view of the road, never on top of it.
- Stop it sliding. A loose phone will shoot across the dash under braking or drop into the footwell on the first corner. A non-slip dash mat is the usual fix.
Where should the phone go on the dashboard? #
- Low in your field of view. The reflection should sit near the bottom of the windshield, where a glance from the road is short. It must never cover the road ahead.
- Not on an airbag cover. On many cars the passenger airbag sits under the top of the dashboard, and anything lying there can be thrown at the windshield or at you if it deploys. Your owner’s manual shows where the airbags are.
- Where it can’t slide into the pedals. A phone that slides off the dash in a hard stop can end up under the brake pedal.
Rules on phones in cars vary by country and state. Many places ban holding or operating a phone while driving, and some restrict devices in the driver’s field of view or anything attached to the windshield. A phone lying flat on the dash isn’t mounted to the glass, but check the rules where you drive, especially before adding reflector film.
Why does HUD mode work best at night? #
The reflection’s brightness is fixed, but the scene behind it isn’t. On a dark road the digits stand out. In full daylight the sky behind the glass is far brighter than the reflected phone, and the number mostly vanishes. Polarized sunglasses can make it fade further.
| Conditions | How the reflection looks | What to do |
|---|---|---|
| Dark highway or rural road | Clear and bright | Ideal for HUD mode |
| Lit city streets | Readable, some glare from streetlights | Usually fine |
| Dusk or dawn | Faint | Test it parked first |
| Daylight | Washed out | Use the normal view in a mount |
| Tinted or coated windshield | Dimmer, sometimes tinted | Move the phone closer to the glass |
Fixing common HUD problems #
| Problem | Fix |
|---|---|
| Two slightly offset images | Normal: the inner and outer surfaces of the windshield both reflect. Moving the phone forward or back usually makes one dominate. |
| Digits backwards | Tap the flip toggle. It switches between mirrored and normal. |
| Digits upside down | Turn the phone so its top points toward the windshield. |
| Reflection in the wrong spot | Slide the phone along the dash; the position depends on your windshield angle and seat height. |
| Too bright on an unlit road | Move the phone further back from the glass, or switch to the normal view on a mount with auto-brightness. |
| Phone slides | Use a non-slip dashboard mat. |
| Screen dims or phone gets hot | Full brightness plus charging plus sun overheats phones. Save HUD for night. |
| Number drops to 0 | GPS is lost, usually in a tunnel or garage. It recovers when the sky returns. |
Pair it with the speed alarm #
A HUD shortens the glance. A speed alarm removes it. Set a limit in Settings before you go, and when you pass it, the mirrored digits turn red at full brightness and the phone buzzes and beeps, then again every ten seconds until you slow down. The beep plays even with the phone on silent, and you can switch it off. The alarm re-arms only once you’ve dropped a few km/h under the limit, so it doesn’t chatter as you hover around it.
It’s a limit you set yourself, not one read from map data, so set it for the road you’re about to drive. How to pick the number, and how it compares with map-based warnings, is in our guide to speed alert apps. A phone HUD won’t show speed camera warnings either; speed camera alert apps explains where those come from and where they’re banned.
How accurate is the speed in HUD mode? #
It’s the same number as the main screen: the speed your phone’s GPS receiver reports, refreshed about once a second and averaged over the last five readings so it doesn’t twitch. With a good fix at a steady speed, that’s typically within a couple of km/h of your true ground speed. Two limits matter at night:
- Lag. Under hard braking or acceleration the display trails reality by a second or two, as explained in GPS speed lag. Your car’s own speedometer reacts faster.
- Signal loss. In a tunnel the phone can’t see the satellites. Instead of freezing on the last value, Speedometer GPS drops the reading to 0 after five seconds without a valid fix.
If the phone shows a slightly lower number than the dashboard, that’s expected. Most car speedometers are set to read a little high; see GPS vs car speedometer.
Battery and heat #
A phone lying flat at full brightness with GPS running gets warm, and a hot phone may dim its screen or pause charging to protect itself. At night that’s rarely a problem, but after a hot day let the car cool before you start, and keep the phone plugged in for anything longer than a short trip. Take it off the dashboard when you park, because a sunny dash gets far hotter than the 35 °C (95 °F) ambient limit Apple designs iPhones for.
Speedometer GPS uses location only while it’s open on screen, with no background tracking. Close the app and GPS stops completely. It needs no mobile data, since the speed comes from satellites.
Frequently asked questions #
Does a phone HUD work during the day? #
Not well. In daylight the scene behind the windshield is much brighter than the phone’s reflection, so the digits wash out. Use the normal view on a mount and save HUD mode for dusk and night.
Why do I see two copies of my speed in the windshield? #
The windshield has two surfaces and both reflect the screen, which gives a faint offset double image. Moving the phone a little usually makes one copy dominate. Cars with a factory HUD use specially shaped windshield glass to avoid this.
Is using a phone as a HUD legal? #
It depends on where you drive. Laws on screens in a driver’s view and objects on the windshield differ by country and state. Keep the reflection low and clear of your view of the road, keep the phone off airbag covers, and set it up before driving.
Can the HUD show navigation or trip distance? #
No. Speedometer GPS’s HUD mode shows your speed, its unit and an over-limit warning. Trip distance and other stats are on the main screen, so check them once you’ve parked, and keep turn-by-turn directions on a separate mounted screen or on audio.
Can I use HUD mode on a motorcycle? #
Most bikes have no glass in the right place to reflect a phone lying flat, so the mirrored view isn’t much use. Put the normal readout on a solid, vibration-dampened handlebar mount instead, and set it up before you ride.