The numbers you quote don't show math, and even if they did, the math is different for each vehicle airflow.
Regardless, efficiency has many properties.
Time means more labour cost + cost of spending another day on the road, plus time not shipping something else.
I sometimes, in my car, drive from Quebec to California. It can take an extra day to make this drive, if I must drive slower due to weather, traffic, or construction.
EG 60mph vs 80mph average (yes, average without stops).
A little more fuel pales in comparison to these costs.
Resistance at 60 is a huge amount higher than at 50.
Go 10% faster if you want, but you'll use 30% more fuel over the same distance.
Numbers are approximate.