Had to take a quick trip to Louisiana today. Filled up, reset mileage and overhead and jumped on I-10. The overhead mpg settled at 18.6mpg and stayed there. I was running with cruise set at 75. About 50 miles later I realized that I had left the tailgate down when I filled up. Overhead still at 18.6. Stopped closed the tailgate and jumped back on I-10 at 75mph. The overhead dropped quickly to 18.2mpg and stayed there the rest of the way. Same driving conditions.

I have had tonneau covers and have heard all the talk about better mileage, etc. I do drop the tailgate at the strip. I never really thought the tailgate down would really have any effect on actual mileage or speed. Maybe I was wrong. A .4mpg difference with the tailgate down is much more than I thought possible.

Just something I saw today.

 

How many miles after putting the tailgate up? Did you park long to put the tailgate up? The overhead mileage indicator seems to plummet at a park. Every little bit of mileage helps. I think I will try putting the tailgate down myself on highway trips unloaded- I just worry about hitting a big bump in the road and having the tailgte slam up and down and possibly off.

 

Actually, these trucks are designed to run with the tail gates up, so your 0.4 MPG diff was only by chance. When running with the tail gate down, it increases drag underneath the rear of the truck. It causes the air to swirl and roll, rather than flow smoothly, resulting in slightly less mpg.

From what I've read, only the new F-150 actually benefits a tiny bit with the gate down, due to such a deep bed.

Tonneau covers apparently offer a slight decrease in drag, improving mpg slightly in all trucks.

I did read about this in a well know auto publication recently. They had actual renderings to help explain the air flow differences. Others have said this as well, but I didn't fully believe it (the drag under the tail gate was not explained & I didn't think of it). I use to drive with my tail gate down on the interstate. I don't have to think about that any more...

 

There was a great thread on this here somewhere. Went into all of the aerodynamic differences. Try doing a search for it.

 

Comstock

I just pulled over on the side jumped out and closed the tailgate. I drove about 75 miles after closing the tailgate. The cruise set same, just hit resume and the same driving condititions. It did take a few miles for the overhead mpg to drop to 18.2, but once it did it stayed there.

It may have just been a fluke, but it did happen. I dont really worry about things like this, it just caught my eye since I was trying to see if my highway mileage has gotten better.

BTW the 18.2 overhead is really 17.1 actual hand calculated this time. My overhead is always .7-1.2 mpg higher than actual and I have had the TSB 18-037-04.

Oh well, just thought I would post what I saw. Thanks for the replies.

 

There was an article in popular mechanics, I think, and they said that in most cases it does not matter much if you have the tailgate up or down, in fact as others have suggested you may get better mileage with the gate up.

Basically the same with box covers....in fact I think it was stated that there was an increase in fuel economy if just the back half of the box was covered....obviously not practical...just an experiment observation.

 

My guess would be the difference in the mileage would be accounted for by the stop, truck idling and the acceleration back up to speed.

I am always surprised how much my mileage drops with idling or lots of stop and go traffic.
Hand calculated will always net me better mileage in these conditions.

Scotty

 

A group at MIT did some research on this and I believe it was Popular Mechanics that printed it. The jist of it was that with the tailgate down you were creating turbulance which actually decreased MPGs. With the tailgate up, there was an air pocket created which enhanced the aerodynamic flow and cause a sort of air bubble in the bed which resulted in a smooth flow of air and less turbulance. Now for the tonnou covers, the roll up soft ones were just as bad as tailgate down because of the flapping of the material, no matter how tight your cover is, caused turbulance. And the rigid fiberglass cover was better than the soft one but less than with tailgate up! I believe it's alot of hype by cover companies to sell their product.

 

Wish I was getting 18.6 @75mph, or even 18.2 for that matter. 4.10's, 70mph, and an ATV in the bed, 13.6 hand calculated this past week heading to Nashville...

 

I believe Cummins also has some data on a dodge quad cab getting better mileage w/the gate up. Not be much , though, but better. Report talks about tire pressure, driving style and much more.

 

My '03 has 43000 mi on the odo. I drive 110 mi a day five days a week on I-44 @ 75 MPH. I have checked several tanks with tail gate off and same with my Flowthru 5er tailgate and miles per gallon doesn't change with or without a gate although I've noticed with the flowthru gate the truck wanders more in a side wind than with the regular gate.........JIM

 

RatherBeMuddin

That 18.2 overhead was really 17.1 hand calculated, that is the best that I have ever averaged on the highway. I hope it means that engine is finally getting broke in(16,000 miles now) and mileage finally going up a little.