PlanetGPS trackers off Amazon. Just plugs into the vehicles OBD. You could have your employees just plug it in at the beginning of their shift and then unplug it if they don’t want you to be able to see them outside of work hours.
I think it’s like $90/yr and the devices are like $30-50, can’t remember what I paid.
You can see the route travelled, and it does mileage reports by date/time. It has all sorts of reports and feed back data. Even aggressive driving, braking, speeding, geofence alerts.. etc.
I don’t track mileage but I got them a few months ago in my vans just to try and improve productivity, found one of my techs doing over 1000km/mo on a leased van outside of work hours. đŸ¥²