Driven to distraction - the mpg problem

The new car has arrived. My Abarth 500c is the bees knees and makes me really acknowledge the shortcoming of the Smart car and its brick suspension. On the way down to London to trade it in I was watching the comsumption all the way to try and do it on a single tank. Mrs Y complained that I was driving too slowly at around 65 mph so she took over and pressed the pedal to the floor. It was a close call but I retook control an hour later for the final leg. On the way back my eyes were glued to the average and acutal mpg on the new beast much to the chagrin of Mrs Y. She says that I am too fixated by the mpg computer . It's an old problem in our marriage but I did point it out to her that the tank seems to go further when I am at the wheel. And for the record the Abarth returned more mpg for a considerably higher speed although I suspect the Luton roadworks played their part.