I took a long road trip in my ranger to deliver some display cabinets I built for a customer. 15 hours on the road in one day. I wanted to see how gas mileage was affected by speed. On the way out I drove 60 mph and got 33 mpg, on the wy back I drove 70 mpg and got 28 mpg.
I suppose the weight of the cabinets, less than 100 lbs, was negligible.
So, driving 10 mph slower saved a pretty decent 5mpg, but added slightly more than an hour to the trip.
FWIW John