A method for a telematics service provider of maintaining a vehicle availability report for a vehicle rental outlet comprising: determining the number of vehicles of a first type within a first predetermined geofence surrounding vehicle ready bays at the vehicle rental outlet based on location data received from on-board units of the vehicles; determining the number of vehicles of the first type within a second predetermined geofence surrounding vehicle cleaning bays at the vehicle rental outlet based on location data received from on-board units of the vehicles; determining the number of vehicles of the first type due to enter the first predetermined geofence from the second predetermined geofence within a predetermined time period based on a stored average time that it takes vehicles of the first type within the second predetermined geofence to be cleaned and to enter the first predetermined geofence; determining a total number of vehicles of the first type that will be within the first predetermined geofence by adding the number of vehicles of the first type due to enter the first predetermined geofence within the predetermined time period to the number of vehicles of the first type within the first predetermined geofence; determining the number of vehicles of the first type due to leave the vehicle rental outlet within the predetermined time period based on scheduled rentals of vehicles of the first type at the vehicle rental outlet; determining whether the total number of vehicles of the first type that will be within the first predetermined geofence is more than, equal to, or less than the number of vehicles of the first type due to leave the vehicle rental outlet within the predetermined time period; and updating the vehicle availability report based on a result of each determining step; characterised in that each data point sent by an on-board unit to the telematics service provider is accompanied with logging data that specifies the number of satellites that the data point was based on and the dilution of precision. determining the number of vehicles of a first type within a first predetermined geofence surrounding vehicle ready bays at the vehicle rental outlet based on location data received from on-board units of the vehicles; determining the number of vehicles of the first type within a second predetermined geofence surrounding vehicle cleaning bays at the vehicle rental outlet based on location data received from on-board units of the vehicles; determining the number of vehicles of the first type due to enter the first predetermined geofence from the second predetermined geofence within a predetermined time period based on a stored average time that it takes vehicles of the first type within the second predetermined geofence to be cleaned and to enter the first predetermined geofence; determining a total number of vehicles of the first type that will be within the first predetermined geofence by adding the number of vehicles of the first type due to enter the first predetermined geofence within the predetermined time period to the number of vehicles of the first type within the first predetermined geofence; determining the number of vehicles of the first type due to leave the vehicle rental outlet within the predetermined time period based on scheduled rentals of vehicles of the first type at the vehicle rental outlet; determining whether the total number of vehicles of the first type that will be within the first predetermined geofence is more than, equal to, or less than the number of vehicles of the first type due to leave the vehicle rental outlet within the predetermined time period; and updating the vehicle availability report based on a result of each determining step; characterised in that each data point sent by an on-board unit to the telematics service provider is accompanied with logging data that specifies the number of satellites that the data point was based on and the dilution of precision.