[MetroMPG]

Found an interesting efficiency test done by an owner of a 1994 Ford F-150 pickup.

His goal: "to finally (we hope) determine the best configuration for fuel mileage" between...

• bed cap on
• cap off, tailgate down
• cap off, tailgate up

1994 Ford F-150 XLT with an 8 foot bed, 5.8L V8, automatic

Note the vehicle year: 1994 - so, no ScanGauge. That created challenges for collecting data.

Fortunately, we have a creative problem solver. What he did instead was to install a $5 used GM Manifold Air Pressure Sensor into a manifold vacuum line, the voltage of which he could monitor while driving.

(I can hear Coyote X now... "COAST DOWN TEST! COAST DOWN TEST!")

Over the same 9 mile stretch of Interstate, on the same day, using cruise control, he & his son ran the truck in the 3 configurations, taking readings from the pressure sensor every 15 seconds to chart engine load.

The results were charted and the values averaged:

(Click through to the owner's site to see the full size version of this chart.)

Not surprisingly, the camper shell run had the highest average load.

However the owner was surprised to learn that the lowest load was with the tailgate up.

His site has more information about the day's events, his MAP sensor circuit diagram, and another chart of the MAP averages. Check it out:

Test to Determine the Relative Engine Load - With or Without a Camper Shell (and with the tailgate up or down)