I had a S-10 for many years and about half of the time I had a cab high shell on the bed. I did not determine a difference in fuel mileage between the shell installed or off and I was habitually checking each tank’s mileage. However the truck was quieter due to the wind not hitting the rear cab window and the suspension was more effective and smoother with a small load on the rear springs. For my purposes the shell was a good fit. Plus I sold it for almost what I paid for it