You can set a travel rate within settings. This allows you to record a value for the depreciation of a vehicle while it is travelling, and fuel costs, etc.
In order to edit this setting, open your company settings. From the 'Users' section, click 'Travel rate'. From this page, you choose whether or not you wish to set an actual travel rate using the check box (above). After this, you can edit the information by clicking in the box and typing, then click 'Save'.