MPG improves on PU with tonneau cover?

The beginning of this month I bought a soft tonneau cover for my 97
Nissan PU, 2.4 l, 4 cyl FI, 62,500 miles. He's a good old boy and looks
almost as good as when he was new. I buy him a present every once in a
while. Last time it was a cat-back dual exhaust, and the time before
that, a keyless entry system.
When I installed the cover, I had about 3/4ths of a tank of gas left.
Since driving it with the cover on, it seemed that my trip odometer was
getting a higher reading compared to my gas gauge than before. I usually
traveled 50 miles between the 1/4 markings on the gas gauge, winding up
with about 190 miles with about an 1/8th tank of gas left when I refill
it. I got to 212 miles with about an 1/8th tank left. When I filled the
tank yesterday and made the MPG calculations from my last fill up, it
seems that I was getting 18.34 MPG. I rarely put more than 11-12 gallons
in the 15-7/8 gallon tank. This time, it was 11.595 gallons.
Before the cover was installed, I was averaging around 16.5 MPG. The
last time I filled up before the cover, I got 16.62 MPG. I haven't
changed my driving habits despite the rising gas prices, or made any
long trips, so it may be aerodynamics.
The next time I fill up, the MPG will be calculated with having the
cover on for the entire period between fill ups, so we'll see what develops.
Watch the mpg drop and burst my balloon!

--

Bill
In Hamptonburgh, NY
To email, remove the double zeroes after @

willshak wrote:
> The beginning of this month I bought a soft tonneau cover for my 97
> Nissan PU, 2.4 l, 4 cyl FI, 62,500 miles. He's a good old boy and looks
> almost as good as when he was new. I buy him a present every once in a
> while. Last time it was a cat-back dual exhaust, and the time before
> that, a keyless entry system.
> When I installed the cover, I had about 3/4ths of a tank of gas left.
> Since driving it with the cover on, it seemed that my trip odometer was
> getting a higher reading compared to my gas gauge than before. I usually
> traveled 50 miles between the 1/4 markings on the gas gauge, winding up
> with about 190 miles with about an 1/8th tank of gas left when I refill
> it. I got to 212 miles with about an 1/8th tank left. When I filled the
> tank yesterday and made the MPG calculations from my last fill up, it
> seems that I was getting 18.34 MPG. I rarely put more than 11-12 gallons
> in the 15-7/8 gallon tank. This time, it was 11.595 gallons.
> Before the cover was installed, I was averaging around 16.5 MPG. The
> last time I filled up before the cover, I got 16.62 MPG. I haven't
> changed my driving habits despite the rising gas prices, or made any
> long trips, so it may be aerodynamics.
> The next time I fill up, the MPG will be calculated with having the
> cover on for the entire period between fill ups, so we'll see what
> develops.
> Watch the mpg drop and burst my balloon!

Some mpg savings is to be expected.
You have eliminated the rolling low pressure drag zone in the bed and
created a boundary layer.
The worst thing you can do is drive with the tailgate down, or off, or
with one of those air gate things.
However, to accurately determine the mpg, you will have to fill up the tank.
"About" 1/8 tank is not good enough. Your gas gauge is not linear, either.