rtate wrote:
When I had a bumper pull I built a rack for my kayak that went over the roof of the pickup and out over the bed. I check my milage pretty closely and did notice a slight improvement with the kayak on top. I am guessing that it broke up the airflow flowing over the cab of the truck down into the bed and hitting the front of the trailer. Actually improved the airflow somewhat.
First off, I agree with everyone who said you shouldn't worry about this if all you are concerned with is gas mileage. Leave the boat home and save a few bucks if that is the total issue.
It all comes down to air flow and rtate brings up a good issue but you will never really know until you pay attention. I tow an Airstream and they are pretty good aerodynamically speaking. I was getting between 10 and 13 miles towing on long trips.I have close to 48,000 total towing miles now. I added a bike rack to the back and noticed that my mileage went down a good 1 mpg even if the rack was empty! The disruption in flow created enough turbulence to cause the change. We still take our bikes because we want to ride them. I always laughed when I saw pickup trucks with their tailgates removed. One person told me it was to increase mileage. The fact is, as was determined by lab testing, this actually hurt mileage because the trucks were designed to have better air flow with the tailgate in place.