I picked up my Toyota Tundra earlier this month, and every time I get a new vehicle I watch the gas mileage closely for the first few weeks.  Obviously I didn't buy the truck for gas mileage, but it's a good thing to know.  I went to buy a tonneau cover for it, and saw the claim that it will improve my gas mileage.  I decided to do a comparison because I had the numbers.

The reason the cover is supposed to help with fuel economy has to do with aerodynamics, and the wind now pushing into the tailgate of the pickup.

The Toyota Tundra is not a fuel sipping truck.  It lacks behind the others when it comes to fuel economy, but I got the Tundra because of it's reliability and resale value.  That being said, I wanted to see if it was on par with what the manufacture claimed it would get.

Without a tonneau cover I drove 500 miles.  It was a mix of city and highway driving.  If I went out of town for an extended trip on the highway, I would reset the fuel economy so I could get an accurate reading of highway miles.   Without the tonneau cover I got 13.8 mpg in the city, and 17 on the highway.   That's just about right on with what Toyota said I would get.

After I got the tonneau cover installed, I reset the fuel economy computer and drove under the same conditions as I had before.  Another 500 miles later and my city driving was 14.8, a full mile to gallon better.  I was surprised to say the least.  On the highway I actually average 18.2.

So yes, in my little study a tonneau cover does make a slight difference.  I didn't buy a tonneau cover for the gas mileage, but it's a nice bonus.