As a mechanic I can honestly tell you a mechanic ( technician now) won't be able to work out a fuel cunsumption rate on checking any vehicle in a workshop. Unless it's losing fuel through a leak the consumption is dependant on engine wear, environmental driving ( rural, urban, low gear use, acceleration, top speed etc) and the load you carry in regards to laden weight. We use fuel cards also for vehicles and they all require a signature, date and time. Surely if it's within the working hours, unless you were filling cans up, or meeting at the garage with your car it's got to be ok? Get the fucker to give you and idea of how much over he's claiming. If it's low, he's at it, if it's lots of litres then some explaination needs to be found. Good luck anyway I'm sure you work hard, most van drivers are certainly keep on the go.