This was something I thought of a decade ago, but which was of such obviously niche interest and dubious legality that I never did anything with it: the idea of using a smartphone display, suitably vertically flipped such that its contents got reflected back from the (typically around 45 degrees) car windscreen, to form a kind of HUD (Head Up Display). And this is exactly what TrueSpeed does here - it's simple and focussed on just one function but the HUD part works very well. With more caveats than you can shake a stick at, but... it does work.

From the Nokia Store description:

• Get TrueSpeed speedometer with HUD and enjoy the cool supercar effect on your windscreen while also driving safer.
• An easy to read dial display with a numeric LCD value. HUD display helps you to concentrate on the road when driving in dark or in bad weather.
• You can also use TrueSpeed to confirm the speedometer accuracy. Did you know that your speedometer could be up to 10% off?

Before I launch into some of the caveats, here's TrueSpeed reflecting off my car windscreen at night, with the information coming from my E7, mounted on a grip mat on the top of my dashboard:

Undeniably cool, and credit to the developers, but do note that:

• The system only works properly if it's quite dark (obviously)
• The HUD has a 'duplicate' effect, due to reflections off the two faces of the windscreen (this may be slightly different for you, depending on the glass coatings and angle)
• There's the distraction of the phone's display too, albeit at a more extreme angle, plus the worry that it might slip off the grip mat, etc. during a corner
• This is a one-display app, you get a visual indication of GPS speed and that's it
• Speed is only shown in km/hour, there's no option to use imperial units (mph)
• Having a HUD at all is going to be distracting, despite what the developers say, not least because the HUD image is, optically, only a few feet away, whereas your eyes should be focussed many tens of yards/metres away for driving - you won't want to be switching your focus backwards and forwards - especially at night, where driving is harder

You can buy TrueSpeed for £1.50 here in the Nokia Store. When first started, the app shows the dial the 'right way around', but a control, top-left, flips it for HUD use.

Comments welcome - would you be willing to use something like this? And do you think the local law enforcement would approve of a HUD, however simple?

NB. As readers have pointed out in the comments below, there are a few other similar applications in the Nokia Store, it seems. I'll do a proper round-up in the future.