If you have an iPhone or an iPod touch you may be interested in an application that just became available on the iTunes app store for those devices. It's called Dynolicious and basically uses the accelerometers in the devices to allow you to record and analyse the performance of your car.
The blurb reads:
Dynolicious is the first automotive performance meter available for the iPhone and iPod Touch. Dynolicious uses the hardware built-in to your iPhone to measure the performance characteristics of your vehicle. No external wires or equipment is necessary, simply slip the iPhone into a cradle or cupholder and go!
Some of the performance metrics Dynolicious can measure:
1/4 Mile Elapsed Time
Unlike other accelerometer-based performance meters, Dynolicious uses the advanced data-handling and display capabilities of the iPhone to make the most of your performance testing. Dynolicious maintains a history of test runs, showing you averages and trends in your results. Dynolicious also allows you to enter modifications performed to your vehicle, and will instantly show before-and-after results to easily identify gains or losses.
Website is http://dynolicious.com/index.html
For the price of £7.50 if this works as described its sounds like a bargain.