I have an allstar fuelcard, which if I used it to actually buy any fuel (no stations near home take fuel cards :/) I would have to declare private mileage which would then be deducted from salary on next month's pay slip.
What I actually end up doing is buying fuel myself then putting the entry into the fuel card website with how much I've spent so my company then pays me the amount of business miles claimed in my pay.
As I mostly drive to one place for work there's no mileage claim for that which is a bit painful but there's no tax liability.