I have absolutely no idea whether this app works accurately, but if you have an iPhone or an iPod Touch, you can get some stats about your car's performance.

It's called Dynolicious and you can find out more here.

For those who care: both the iPod Touch and the iPhone have accelerometers which are normally used to detect orientation - the display will rotate depending on how the device is held. Since the 2.0 firmaware update and the platform being opened for developers, a lot of interesting applications for these have appeared. These range from a spirit level (one of the demo apps supplied with the SDK) but racing games (you use the iPhone as a steering wheel - tilt to steer) to artwork apps which clear the screen when you shake it.

This is an interesting use for these devices.