Car mileage is an obvious example for personal data tracking, especially because it is a key indicator of the overall health of your vehicle. Put in the total fuel and total cost readings from the pump, and tick the 'tank full' checkbox to activate the mileage function calculation.
This rTracker example shows the use of the 'Start with last saved value' option on the odometer value; this way you will usually only need to change the last few digits of this number when entering data for a fill-up.
In the Car tracker, the total distance travelled and price per unit are calculated and logged. I use the 'total cost' field to log service expenses as well, and the 'notes' item is a full textbox -- perfect for logging longer trips, service details, or other issues that will help in understanding your car's performance over time.
With rTracker's flexibility, it is easy to add a conversion for 'miles per gallon' or similar if the fuel pumps in your country report litres.