[elhigh]

It so happens, I have here in front of me the log of my pickup. It includes readings before and after the cap was installed.

The cap is a roofline-height model, so it doesn't stick up in the wind. It also isn't especially heavy, about 150-200 lbs. I can install it by myself.

Before cap: best mileage = 28 mpg. Avg mileage ~ 27.
After cap: best mileage = 37.4mpg. Avg mileage ~ 30.
Cap off for several months a few years ago:
Best mileage: 28.5mpg. Avg mileage ~27.75mpg.

All of these are in excess of the EPA's pie-in-the-sky estimates.

I don't drive especially fast, but people aren't shaking their fists at my dawdling little truck, either. Most of my driving is highway, the vast majority of it unloaded, in all weather.

In October, on its 20th birthday, the truck delivered 29.54 mpg. I wasn't even trying to beat the EPA, it just happened.