Bought a new 2014 Toyota v6 dbl cab 4x4 - I am thinking of getting a tri-fold hard Tonnau cover for it and was wondering if anyone noticed better mileage from having one on their Tacoma or any other pick-up truck?
It seems only available behind a pay wall, but the National Research Council of Canada did wind tunnel testing of pick up tailgates. The short form of that is to keep in up for most trucks.
They also looked at a partial tonneau cover. Why partial? Because GM had a patent on the use of a partial one because helps with aerodynamics. I had one on the Ranger because of this paper. Can't say how much it helped at this point since the data is gone, and I no longer have the truck.
So tonneau cover will improve aerodynamics. Depending on the commute, that might not equate to improved fuel economy. Even then, the efficiency gains likely won't cover the cost of the cover. If you want to cover the bed for other reasons, it can be a bonus.
I know this thread is old now, but testing has been done by multiple sources that says a complete bed cover makes fuel economy worse (so does removing your tailgate with an open bed). I think there was a Mythbusters epsiode, and a government agency of some sort did some testing, and then Ford showed off their new F-150 with some wind tunnel testing with Autoblog.com.
Virtually all trucks behave the same way as the Ford in the video in spite of no fancy spoiler specifically designed to reduce drag; it's something to do with a pressure zone being created in the truck bed that deflects air OVER the truck instead of maintaining turbulence while you drive.