Every car produced after 1996 is fitted with an On Board Diagnostic (OBD2) port. It’s used by technicians to check the health of your car and to see what parts are failing. Nowadays you can do more with your OBD2 port, for example use it while driving to show more information about your cars sensors than your normal dashboard does.
Browsing the iPhone App Store I hit on this gem by accident. I found an iPhone application called DashCommand and started reading. It turned out that you needed a device to connect to your OBD2 port in your car. This device creates a connection to your laptop of iPone. Many are available but I bought the PLX Kiwi Wifi, because it creates a wireless connection. So my setup is an iPhone, a device to connect to your OBD2 port (PLX Kiwi Wifi) and a piece of software (DashCommand), but of course you can use many different setups.