Fleet app has two security group, 

Fleet\User which enable user to see his only vehicle by assigning it as driver. 

Fleet\Administrator have full access to Fleet. 

So to give some users access to see all vehicles you can duplicate Fleet/User group and name it "User All Vehicle" and go to Access Rule tab and delete all record rules to allow users with this group to see all vehicles, vehicles contracts, odometer and services. Once this group created, Refresh your browser and Go to the user form and you will find the fleet security at the bottom with three checkboxes and assign the user to new create group  "User All Vehicle". 

By doing this you will have three groups for Fleet and since the new created group not inherited from another fleet group, the fleet access in user Form will be shown as checkboxes.