Written by Muneeb Rana · Last updated 26 October 2026
Quick answer: A HUD display for car use comes in three forms: a factory-fitted head-up display, a plug-in aftermarket unit that sits on the dashboard, or a phone app in HUD mirror mode. Phone apps cost least and need no wiring, but are dimmer by day. Plug-in units are brighter. All should be set up before driving.
TL;DR
- Three options: factory, plug-in, phone app.
- Phone HUD apps: cheapest, dim by day.
- OBD units read the car's own speed.
- GPS units work in any car.
- Mount securely, never block your view.
Phone app as a HUD display for car
Apps like Speedometer PRO mirror the speed so the windscreen reflects it. You already own the hardware. Downsides: brightness limits in sunlight, phone heat and battery use. See how to use HUD mode.
Plug-in OBD HUDs
These read speed and other data from the car's OBD-II port (standard on US cars from 1996). Because they use the car's own data they inherit its over-reading speedometer.
Plug-in GPS HUDs
Stand-alone units with their own GPS receiver work in any vehicle, including older ones without OBD. Accuracy is like a phone's.
Comparison
| Type | Cost | Daylight | Speed source |
|---|---|---|---|
| Phone HUD app | Low | Weak | Phone GNSS |
| OBD plug-in | Medium | Good | Car sensors |
| GPS plug-in | Medium | Good | Unit GNSS |
| Factory HUD | High (option) | Best | Car sensors |
Legal and safety notes
Rules on items on the dashboard and screens in view of the driver vary. Do not place anything that blocks the view or airbags. NHTSA treats visual-manual phone use as distraction; set up first.
Aftermarket HUD units
Besides phone apps, there are plug-in HUD units that take speed from the car's OBD-II port or their own GPS and project onto a small reflector or the windscreen. OBD units read the car's own speed signal, so they match the dashboard; GPS units match true ground speed more closely. Both need a stable spot on the dash and, like phones, shouldn't be adjusted while driving. Check reviews for brightness in daylight, which is the most common complaint.
Which to pick
Try a phone HUD app first; if you love it and drive mostly in daylight, a plug-in unit may be worth it. Our HUD speedometer apps list compares apps.
Related: HUD speedometer guide · How to use HUD mode · What is a HUD? · Best HUD speedometer apps · Speedometer HUD mode for your windshield · Speedometer for car: your phone as a second display · All speedometer guides · GPS speedometer guide
Get Speedometer PRO on Google Play · Download on the App Store · Set it up before you move; never handle your phone while driving.
Sources
- Head-up display (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
- Distracted Driving (NHTSA): nhtsa.gov (accessed 26 October 2026)
- Speedometer (Wikipedia): en.wikipedia.org (accessed 26 October 2026)
FAQ
What is the cheapest HUD for a car?
Using a phone HUD app you already have costs little or nothing, though daylight visibility is limited.
Do HUD units work in old cars?
GPS-based units and phone apps do. OBD units need an OBD-II port.
Can I add a HUD to my car permanently?
Plug-in units can stay on the dash. Factory HUDs are generally not retrofittable.
Is an OBD HUD more accurate?
It shows the car's own speedometer value, which usually reads slightly high; GPS shows an estimate of true ground speed.
Why can't I see my phone HUD in the day?
Sunlight overwhelms the reflection. Turn brightness up or use the normal display by day.