Given information about the distribution of the sizes of fields he mows and the field efficiencies of the two mowers for the various size fields, Tim could compute weighted-average estimates of the AVCs.
Tim will have to decide whether the effort to convince his wife is worth it. After 45+ years of marriage, I have learned that deploying economic logic is not always successful.:)
I have never calculated my efficency. I would guess it's about 50-60%. I do try and make as many long straight runs as possible. And once the strip I am mowing gets narrow enough I stop cutting across the ends and loop around and go down the other side. This saves the time of backing or 270*ing the corners at each end.