There are two things to look out for when you are tracking your expenses:
1. Your kilometers.
2. Parking fees.
Tracking Gas Expenses:
From the time you first leave the office, to the time you return, you should track the kilometers on your car. Make a note of your car's kilometers before you leave for the first job site, then calculate how many kilometers you have driven once you return.
Here's a video on how to track your gas expenses: