OVERVIEW:

With fuel economy an increasingly important concern in this day and age of escalating gasoline prices, industry-leading fiberglass truck cap and tonneau cover manufacturer Snugtop undertook an “apples to apples” comparison test to see how the aerodynamic enhancements offered by caps and lids affected gas mileage, compared to stock open-bedded pickups. 

The concept was to take three similarly-equipped late model trucks (a Chevy Silverado, Dodge Ram and Ford F-150) and test them under similar conditions, with and without caps/lids. 

METHODOLOGY

In a perfect world, the tests would have been conducted in a totally climate-controlled environment, at speeds more in keeping with highway travel, and robots performing all the tasks.  But since that was virtually impossible, Snugtop did the best they could under the circumstances.

To avoid the pitfalls of trying to perform tests on city streets or highways, Irwindale Speedway’s facility was offered by General Manager, Bob DeFazio.

Preliminary testing showed that a speed of 45 mph was about as fast as a factory-stock truck could be driven consistently on Irwindale’s 1/2-mile oval without being affected by tire scrub in the corners, etc.  And to eliminate a variable of driver gas pedal actuation, cruise controls were employed exclusively.

One area that could not be tightly controlled was the “line” each driver took, however all were instructed to stay close to the line that’s painted in the center of the track.  And the fuel economy was calibrated from actual miles driven, so if one driver had a slightly more circuitous route around the track than the others it didn’t matter.

Prior to running the laps, the three trucks were taken to the Automobile Club of Southern California (AAA) Test Facility in Diamond Bar, CA, and their odometers and speedometers calibrated on a dyno by Automotive Research Specialist Paul E. Keen, Jr.

The trucks were fueled using the same parameters as AAA uses for its mileage tests, according to the facility manager, Steve Mazor; fill to the “click” three times in succession. All refueling was done at the same Irwindale Raceway pump –which reads fuel use to three decimal points—and by the same person (Joe DeFazio).

Each truck was equipped with one of Irwindale Speedway’s transponders, so lap data was fed into the track’s scoring system and monitored by veteran NASCAR official Jack Buchanan.

THE VEHICLES

• 2006 Chevrolet Silverado Crew Cab with a 5.3L Vortec V8, automatic transmission, and about 13,600 miles on the odometer.  AAA calibration showed that for an indicated 5.0 miles traveled the actual dyno-certified distance was 4.9178 miles (correction factor of 0.98356).
• 2006 Ford F-150 Extra Cab with a 4.6L Triton V8, automatic transmission and almost 8,000 miles on the odometer.  AAA calibration showed that for an indicated 5.0 miles traveled the actual dyno-certified distance was 4.9574 miles (correction factor of .99148).
• 2004 Dodge Ram 1500 SLT Quad Cab with a 5.7L Hemi V8, automatic transmission and about 15,100 miles on the odometer.  AAA calibration showed that for an indicated 5.0 miles traveled the actual dyno-certified distance was 4.9280 miles (correction factor of .98560).

TEST RESULTS

The tests were conducted on Thursday, August 3, 2006 between the hours of 11:30 AM and 3 PM. The sky was overcast and temperatures remained fairly consistent in the 78-80° range. The wind was relatively calm.

First in line was the Dodge, which was filled and sent out on the test with 15,155 miles on the odometer.  It was equipped with a SNUGTOP cab-high cap.

The Silverado was next, and hit the oval with 13,646 miles on the odometer and a SnugLid SL covering the bed.  Finally the Ford, also equipped with a SnugLid fiberglass tonneau cover, hit the track, fully tanked and with 7,998 on the odometer.

After doing the requisite 100 laps with the cruise control set at 45mph the drivers “pitted” and were topped of per AAA procedures.  The Dodge got 19.38 mpg, the Silverado scored highest at 23.06 mpg and the Ford squeezed out 21.30.

The “pit stop” also included removing the SNUGTOP cap and SnugLid tonneaus from the vehicles.  Off they went with Irwindale Speedway officials Lester Boyer, Mike Atkinson and Joe DeFazio at the controls.

When they returned and got topped off the Hemi-powered Dodge’s mileage had dropped to 18.06, compared to 22.60 for the Chevy and 21.42 for the Ford (which had been lapped and inadvertently came in at 99)

Incidentally, Irwindale’s race central calculated that the “fastest” truck averaged 45.241 mph for the 100 laps and the slowest 44.496 mph.

CONCLUSION

What did we learn?  Under fairly stringent testing procedures we quantified fuel savings as high as 1.3 miles per gallon.  And that was at a speed of only 45 miles-per hour.

We know that the results of the testing would have shown even better improvements if the baseline had been 60 mph instead of 45.  Engineers tell us that aerodynamic drag increases with the square of speed.  So even though the increase in speed from 45 mph to 60 mph is about 33%, the drag would increase exponentially to just over 75%.  Now, there’s a whole lot more involved here than simply mitigating increased drag through the aerodynamic enhancements offered by a truck cap or tonneau cover, but the bottom line is that highly credentialed engineers will agree that a testing at a higher speed would produce additional increases in fuel economy. They just can’t assign an exact number to the gain.

SNUGTOP has received solid feedback from customers who have typically claimed increases of 1.5 to 2 miles per gallon after equipping their vehicles with the company’s caps and lids.  This pretty much corroborates the test results.

Of course, results will vary by vehicle condition, its equipment and the type of driving done.  But it’s a safe assumption that under normal circumstances it’s possible to save hundreds of dollars annually in fuel costs by covering up that empty bed with a cap or tonneau cover.