How you measure business travel and monitor any improvements will depend upon the targets you set. Your monitoring should include all business travel, including any fleet vehicles.
What can be measured?
It is likely that most of this information is available via your HR department and just needs to be re-formatted slightly to fit with your monitoring requirements.
It would also be useful to monitor the use of tele conferencing/ video conferencing and by whom. Again, depending on the system that your organisation has in place, you should be able to monitor usage by department. Alternatively, you could do a spot check survey about tele/ video conference usage to gauge how well used the facilities are.
From this information you will be able to find out if some trips could be made by other modes rather than the car and whether trips could be combined and how efficiently fleet vehicles (company cars, vans and delivery lorries) are being used.
You should liaise with the Fleet Manager and HR to gain access to this information on a regular basis and to work with them to identify how to improve efficiency and reduce unnecessary business mileage.