Customizing your truck is about understanding the physics that govern its performance as much as it is about understanding which upgrades support your lifestyle and primary vehicle use. When it comes to accessories that change the aerodynamic profile of your truck, those same principles apply, even if they don't directly affect engine systems. There are a lot of ways to decrease drag and improve fuel efficiency, but there's also a lot of misinformation out there. Luckily, the reports that hard tonneau covers like the Bak Bakflip MX4 folding cover decrease drag and increase gas mileage seem to be true, the phenomenon has been confirmed in wind tunnel tests.
How Many Studies Support Improved Mileage?
The only wind tunnel test that showed improved gas mileage was the 2007 study by the Specialty Equipment Market Association. That study did not separate hard and soft covers but did include both. In fact, it used 13 separate designs. It also used four trucks, one flagship model from each of the four major American-owned truck manufacturers. Each truck tested a different build style.
- GMC Sierra
- Chevy Silverado
- Dodge Ram 1500
- Ford F-150
The study found an average 5.7% reduction in drag and 1.8% increase in gas mileage. It's not a lot, but over a few years of vehicle ownership, it adds up to dozens gallons of gas for most trucks. Unfortunately, the individual results are not available to compare hard and soft covers, but the study did report at least modest gains by each truck. So what about replication? Why would only one study show improved fuel efficiency?
There have been a few attempts to replicate the 2007 findings, but they have notably used only a single truck each. Additionally, none have tested in a wind tunnel, only in field conditions where other factors could have influenced fuel efficiency. It's also unclear whether attempts like the one on the TV show Mythbusters were using stock trucks or if they had aftermarket accessories that changed the truck's performance in addition to the tonneau cover. Lastly, no follow-up attempt to date has used hard covers.
If you're about to give up on a search for tonneau covers near me because of these results, it's worth keeping in mind that none of them actually replicated the wind tunnel experiment, which is what would be needed to rule out its findings. They used entirely different experimental designs and far less formal methods of recording and analyzing data. In the case of the Consumer Reports study, there's no control for speed in each trial and no clear repeat trials to corroborate the data, although the study was otherwise structured in a straightforward way.
There's a chance Bak tonneau covers won't have any effect on your fuel efficiency at all, but no support in any study for the idea that a hard tonneau cover will decrease your fuel efficiency by increasing drag. Soft covers, on the other hand, have been shown to cause reduced mileage in a couple of cases. Although those experiments were less rigorous in some ways than the ones that showed improved mileage, your best bet is to go with a hard cover if you're interested in fuel economy.