Most of those people are in exempt positions such as commissioners and their deputies. Some are supervisors such as the street superintendents who have to be able to come in on a moments notice,at all hours, to any location in the city, mostly during severe weather.
They also pay social security tax on a fixed amount ( all though I don't know the formula) as the use of the vehicle is considered income.
Some of those guys live outside the city as that particular department is exempt from residency.
I think, personally, that some of them don't need vehicles, those that do don't need the loaded top of the line vehicles, such as the Tahoes they drive.
Just my feelings.