So with the tailgate down you can virtualy say that the rear end is lighter thus causing less traction at the rear wheels thus causing my brain to start to slip after al these posts

 

On my PSD every time I took the tailgate off to fetch my overhead camper (70 miles away) I always got better mpg, a good 2mpg more than if the tailgate was up or in the down position.
I even tried an experiment & kept the tailgate off for 2 weeks & yes I did get better mpg but 2 mpg is not that big of a deal, as that PSD avg 18 mpg most of the time with the tailgate up.

 

I really don't think you can compare the aerodynamics of a truck to an airplane wing considering that the area of high pressure is created in the bed of the truck and not under it. However, to answer your question, a lighter vehicle does get better gas mileage.

 

Here is an article from the scanguage website regarding the tailgate up/down question. Sorry about the length of the post here but apparently you can't post a link. Have a wonderful day.

Tailgate Up/Down and Gas Mileage - May 2004



With gasoline going ever higher in cost, I've noticed people putting more effort into
squeezing some extra miles out of a tank of gas. In particular I've noticed more and more
people in pickup trucks driving around with the tailgate down. I believe this is being
done with the belief that the air can flow easier through the bed of the truck and, with
less wind resistance, the gas mileage will go up.

This is a good theory. Without an easy way to measure it, it stays just a theory.

Another theory I have seen is that the mileage will be higher with the tailgate up
because air going over the cab creates a vacuum in the bed when the tail gate is up, causing
the air to go over the bed which reduces drag. What should I believe?

I recently got hold of a new device called a ScanGauge that can attach to the OBDII connector
in my truck and read out the gas mileage in realtime. Finally, a way to test the theory.

To get good scientific data, I needed to run the truck over a measured distance in as
close to identical conditions as possible, with one exception - once with the tailgate up
again with it down. To make the test even better, the test should be run in both directions
in order to cancel out terrain and wind effects. The test had to be done at a constant speed
within a few minutes of each other.

I chose a recently completed 6 lane freeway which still had very little traffic. The speed
limit was 65MPH. By traveling in the slow lane at 65MPH using the cruise control to keep the
speed a constant 65MPH, I was able to maintain this constant speed over a 5 mile coarse.

Before starting the test, I got the truck up to 65MPH and set the cruise control. I set the
ScanGauge in the CURRENT TRIP mode. This mode allows a reset to be done at any time so I could
accurately start the measurement when I passed a milepost. The fuel economy display showed
the average MPG starting from the time the RESET was pressed.

The test was started when I passed milepost 15 and completed at milepost 20. I then went
back the other direction pressing RESET at milepost 20 and noting the trip MPG at milepost 15.
The average of these two was taken for each condition, tailgate up, and tailgate down.

The results were surprising.

• TAILGATE UP = 17.6 MPG
• TAILGATE DOWN = 17.7 MPG

By surprising, I also mean disappointing.

Did the mileage go up with the tailgate down? Yes, but the amount is so small as to be
statistically insignificant. A single gust of wind during one of the 10 mile runs could account
for this much difference.

If someone out there has a removable bed cover, I would like to see this kind of test run
for it on and off. For someone with more ambition and one of those caps that blends into the
truck, this kind of test could be even more revealing. Does the cap help or hurt MPG?

For now I plan to keep the tailgate up. I feel more vulnerable with it down. I don't think
it would fare as well as the bumper if someone decides to see what happens when they run into the
back of a truck.

For more info (and more scientific data to sort through), the following conditions existed:

• Speed variation (from the ScanGauge) min=64, max=66
• Coolant temperature variation (from the ScanGauge) 198F to 204F
• Intake Air Temperature Variation (from the ScanGauge) 98F to 101F
• Outside air temperature (from the trucks outside temp) 92F.
• Vehicle: 2001 Ford F150 Extended Cab 4x4
• Vehicle Weight = approximately 5500 lbs
• Drivetrain: 5.4Liter V8 with 4 speed automatic

 

so if it isnt true then why does nascar truck series have a flat bed. the tailgate down the wind comes over and right out the tailgate i have checked milage 4 different times exact same route same weather conditions and i got 1-1.5 better with the tailgate down. but now i have a cover and it does help with milage not much but it does help. it is simple physics less resistance for the air to travel

 

Sorry to be blunt, but I think the dumbest anyone can be is to use cruise control to improve MPG - any car I ever owned or drove usually pounded the pedal quite a bit when it would have to accelerate. Especially this V10

While that guy that did the scientific test was trying to hold speed, the cruise control could be to blame for the almost exact same numbers, just based on terrain. When the motor is obviously pushing a lot more with cruise on than you would with your foot, the MPG is obviously going to suffer.

 

I get into this argument all the time here and at work.

For the record: 86.5 Nissan v6, 99 7.3 F350 PSD 4x4 LB CC, 01 F250 V10 4x4 SB CC, 05 F350 V10 4x4 LB CC.

3 tank fulls with gate up, 3 tank fulls with gate down I did manual math and not enough diff to say either way.

My personal testing is mixed terrain, mixed city highway, and various speeds associated with typical commuting.

One "test" was out to El Paso 712 mile with gate up and trip back with gate down. Not fair cuz the trip out was heavy and trip back was empty.

My personal tests tell me that MY trucks do NOT get any significant extra MPG with the gate down. The addition of the tonneau cover or cab height cap seem to get 1-2 extra mpg.

Nope not at all scientific (sheesh me being a test Officer and all)

I will tell you I have seen down tail gates come flying off on bumpy roads and it aint cool dodging a big sparking sliding hunk of steel at 75mph! The last time this happened to me was in Colorado last year heading to Sturgis on my bike, with wife in trail in the truck, she near ate me as we both did the escape and evade manuvers! Be carefull out there! If your going to run with it down, don't, take the damn thing off!

 

[QUOTE=txag]Lift is caused by high pressure on the underside of an airplane wing.

Airplane wing lift occurs from high pressure OVER the UPPER wing

 

How's this ?

http://www.aerospaceweb.org/question...cs/q0005.shtml



You can be a pretty busy reader with a quick google search on the topic. There are millions of hits depending upon how you enter the search.

 

Actually the high pressure is located on the bottom of the airplane wing. The top of the wing is low pressure. I drove my truck on a 300 mile trip. Out one day, same route in reverse the next day. Going I left the gate up and coming home I put it down. No difference in mpg.

 

I think the kicker is the turbulence in the bed and behind the cab - the tailgate doesn't effect it that much - but at the risk of being redundant, a cab-height cap does help. Because all the turbulence in the bed is gone.