The Distance Traveled Report lets you pull out the mileage for the given phone (or group) for the selected period (date range):
Here is how:
Navigate to the 'Phone Group' under the 'Reports' section.
Select the 'Distance Traveled Report'
Select the Group, Date range, and the format and hit 'Get Report'.
Here is a sample report:
The Starting Miles indicate the odometer reading at start date (of the reporting period) and Ending Miles indicate the odometer reading at the end date (of the reporting period). Traveled Miles indicates the distance traveled by the device for the selected period.