As everyone suggests, you can have a logbook. Two things here:
1. Own vehicle
2. Through contractor
Own Vehicle
If it's your own vehicle, the logbook can be maintained, and you can ask the driver to get a signature from the traveler. Additionally, you can establish a partnership with a petrol station to help monitor the actual kilometers done with every refill. Furthermore, you can keep a logbook at your security checkpoint to track the average kilometers driven based on the location mentioned.
Through Contractor
If it's a contractor, installing a meter will assist you better than any other method.