They use Imperial Gallons, and they lie.
VW could say my Golf gets 93 MPG because when cruising on level ground at about 75 km/h it uses about 3L/100km (according to VCDS).
VW doesn't have to lie to sell cars.
When people think of what mileage a car gets, what they really want to know is how much fuel they will have to buy to drive the thing around under normal conditions. For example, drive it 10,000 miles and figure out how much fuel is going to cost.
When a manufacturer gives some inflated number, that is only true under very specific circumstances, which will most certainly not be achieved the majority of the time, that's lying. " It gets 100 MPG!* "
*When coasting slightly downhill with a tailwind... Otherwise it's just a bit better than a Dodge Ram Truck.