mileage better w/ tailgate UP????

[ntsqd]

snip......
You have never seen a Suburban or Blazer with the wing on the back? Its just a little spoiler that helps keep the rear window clean by redirecting airflow..

It does that and more. By redirecting air into what would otherwise be a low pressure (partial vacuum) area they reduce the total air drag of the vehicle. Think about how much force even a couple psi over the whole rear of a K5 or Sub would be. That force would be resisting the vehicle trying to go fwds.

The real solution is to use a tonneau cover. It eliminates the large cavity of the bed.

There is a lot of anecdotal evidence that tailgate down or 80's mesh gate improves gas mileage, but how many real tests have those people done with the same load, same road, same wind conditions, etc? Few people are patient enough (several tanks of gas) or scientific enough to really know what things affect their mileage and by how much.

If you can keep the air in the bed from leaking out (see below) then the air coming over the top of the cab will actually pressurize the air in the bed, creating a 'tonneau cover' of sorts.

A while back there was a rumor circulating that Ford had done some wind tunnel tests looking at Coefficient of Drag on pick-ups. They reportedly found that tailgate up had less drag. Witness that most late models have subtle, but extensive baffling around the tailgate to keep air from easily flowing thru the cracks around the edges. I know Ford went as far on the Rangers to put in rubber lip seals of a sort.
The missing part of the rumor was any seemingly legit documentation to the claim. Might be out there, but I never saw it.

 

[thezentree]

They used a computer hooked up to the vehicle's (ford explorer) computer to make the calculations in fuel economy. But...if you continued to watch the show, they drained out all the gas again, filled them up with 5 gallons, one with ac on and windows up, and one with windows down/no ac...and drove them around the test track again. This time however they didnt use a computer to measure the mpg, but just drove both around the track until they shut off. The one with the windows down went 40 more laps. They "busted" the myth in favor of windows down gets better mpg...
-Harrison

I think that may vary from vehicle to vehicle. I know with my Yukon, if I'm driving at highway speeds, the torque converter won't lock up until a higher speed with the windows down. A few summers ago, I was driving down Highway 12 in the Outer Banks, running from a hurricane. I was riding with my windows down trying to be more fuel efficient, and I actually couldn't keep the TC locked up with the windows down because the wind was so strong. I realize this is an extreme case, but the TC locked up fine with the windows up and the AC on.

So for how much of a difference that makes, I would imagine that my truck is probably more fuel efficient with the AC on and the windows up at highway speeds. I could be wrong, but that makes sense to me.

However, I bet in city traffic, windows down is much more efficient.

 

[dyeager535]

At least on smaller cars you are absolutely correct in regards to AC...slow vehicle speeds, windows down is better. Once you start going faster, the drag from the open windows is harder on the engine than the AC compressor.

On a big truck/engine I have a feeling the window/AC argument is less of an issue, as is the tailgate/airflow problem at lower speeds.

 

[blazinzuk]

for those who wish to know about top on or off, I got better mileage with it off. Mine is a 73 with a big lift and at the time 38" radials. The mileage was checked with a GPS and I just filled the tank as full as I could get it to calculate how much fuel I used. Trips were taken on different days very very similar weather conditions. Very little if any wind. I got 10.2 with the top on and 10.5 with the top off. Yah virtually the same. I have toyed with the idea of trying to make my truck more areodynamic with some sort of nose peice but the thing that sabotages the whole thing is how high my truck is. Hopefully with my new motor I will go back to the 10 mpg