Does a LEER Truck Cap Improve your Gas Mileage?

 

You've acquired your brand-new pickup truck, and after accumulating some miles, you're analyzing your fuel economy figures only to find them underwhelming. It's no secret that pickups inherently exhibit poorer fuel efficiency compared to passenger cars due to their size, weight, and typically boxy aerodynamics but the actual consumption you're experiencing might be more severe than anticipated. One common aftermarket modification to address such concerns is the installation of a LEER truck cap or a LEER tonneau cover, which encloses the truck bed. The rationale behind this is straightforward: by covering the open bed, you reduce aerodynamic drag, theoretically improving fuel economy. We wrote another article about the fuel impacts of a tonneau cover, but does a bed cap also mitigate the inherent aerodynamic penalties of a pickup bed enough to meaningfully enhance MPG?

 

The technically accurate response is it depends on multiple variables; including vehicle geometry, driving speed, atmospheric conditions, and driving style all of which influence aerodynamic drag and fuel consumption. Moreover, comprehensive, instrumented testing under controlled conditions remains relatively scarce, and real-world user reports show significant variance. However, aerodynamic principles and available empirical data converge on a consistent conclusion: the effect of bed caps on fuel economy is marginal at best.

 

To unpack this, consider detailed aerodynamic testing performed by The Fast Lane Truck, a respected automotive testing outlet. Utilizing a controlled, repeatable methodology, they conducted fuel economy assessments on a Ford F-150, both with and without a LEER truck cap. The results demonstrated a marginal improvement of approximately 0.1 miles per gallon (mpg) when the bed cap was installed. This negligible gain aligns with aerodynamic flow characteristics observed in wind tunnel testing of pickup trucks.

From a fluid dynamics perspective, the aerodynamic drag on a pickup truck is dominated by the wake generated behind the vehicle's cab. The sharp vertical trailing edge of the cab causes airflow to separate abruptly, creating a turbulent wake region characterized by vortices and low-pressure zones. This flow separation occurs upstream of the truck bed, meaning that covering the bed with a cap does little to alter the primary flow detachment and resultant pressure drag.

 

 

Further insights come from wind tunnel experiments using scale models with various truck cap geometries. A LEER truck cap with a fastback (aerodynamically tapered) profile can reduce the formation of a high-pressure stagnation zone inside the truck bed by encouraging smoother airflow over the vehicle's rear. However, this benefit is offset by the generation of a low-pressure vacuum region downstream of the tailgate, effectively increasing the overall drag coefficient. Conversely, a flat-backed LEER bed cap tends to induce a persistent low-pressure zone over the entire rear surface, analogous to flow separation phenomena observed on other bluff body vehicles such as hatchbacks, delivery vans, and tractor trailers, which suffer from increased aerodynamic drag.

 

Quantitatively, the drag reduction afforded by a LEER truck cap is often measured in fractions of a percent, insufficient to produce a substantial improvement in fuel economy under typical highway driving conditions. 

In conclusion, while a LEER truck cap offers practical advantages in terms of cargo security and weather protection, it does not constitute an effective aerodynamic modification for significantly enhancing pickup truck fuel economy. Truck owners aiming to reduce fuel consumption should prioritize driving techniques that minimize aerodynamic drag and rolling resistance, such as maintaining moderate speeds, minimizing aggressive acceleration, and reducing unnecessary payload weight.