I must have picked up on your statement,
"At normal motorway speeds the engine has to do a lot of work and that means it has to consume a lot of fuel."
Which I do not agree with.
We all use cars, they use fuel, how much only matters to the driver, or not in my case. As long as my car is in the ballpark for normal use I am happy, if not I will get it fixed. To date my car has varied between 37 mpg and 45 mpg depending on use. I have not done a long journey yet but will be doing an 800 mile round trip in a couple of weeks that will give an indication of motorway consumption.