I am 1/3 of the way thru a 7,500 mile trip (Ottawa, Ontario - Edmonton, Alberta, - Mount Rushmore, Chicago and return to Ottawa). Having kept meticulous records I can say that I have averaged 10.3 mpg (imperial).
Along the north shores of Lake Huron and Lake Superior (very, very hilly with many changes of elevation) I only achieved about 8mpg. I am now on the prairies, which is so flat you can see your dog running away for three days, I now get about 15 mpg. The average is 10.3.
I drive about 90 kph (~56 mph).
In "The Pas" Manitoba gas was $1.69/litre and in Alberta it is $1.08. Now that I am in the flatlands and getting better mileage gas is cheap!