I don't mean to barge in here but I would imagine there were more variables involved that everyone is overlooking. I've had my fair share of rentals/loaners etc.
One in particular was a Nissan Versa hatchback with the puny 1.8L engine & 4 speed auto (remember, 2L is a pop bottle!) and my daily driver is a '00 Durango R/T full time AWD that averages 15-19L/100km.
Anyway, the Nissan averaged 11.8L/100km, and I had it for a week and filled it up twice in three days. The published figures were less than 10L/100km around town and I drove mostly highway at or under 110km/h. Mid week the service light came on for an oil change which I told Enterprise about, and they said to keep driving it for the three more days. I decided to check the tire pressures as one looked low and sure enough, all four were between 23-27psi. I pumped them up to 34 and my next tank I averaged 8.8L, a 20% improvement! Oh, I also had to purchase washer fluid as the tank in the car was BONE DRY. At least I got an additional discount after I had to spend my time, effort and money to maintain their car! I wasn't impressed.