improve gas mileage 1

[bigTomHanks]

Hi,
I am interested in making a minor modification to my truck to help obtain better gas mileage. I can either put my tail gate down or I can buy a bed cover. Which would help me achieve more savings in gas mileage? I know the initial investment of a bed cover is going to probably overshadow any savings, but I am also wanting to get one for cosmetic reason anyway. I am concerned because for 4 months I have to commute an excessively large amount(12000 miles total over the 4 months) and want to get better mileage. Thank god this situation is only temporary.
bigTomHanks

 

[IRstuff]

Are you even going to be able to go fast enough for it to make a difference?

TTFN

 

[bigTomHanks]

IR
I'll be driving all of those miles on the interstate at 70 mph. How fast would I need to go to make a difference?
bigTomHanks

 

[IRstuff]

Not sure; I live in LA, so most of our interstates don't go more than about 35 mph during commute hrs ;-)

My guess, for what it's worth, is that opening the drop gate might be better. Although, you might also need to put a diverter at the back end of your cab in any case.

Of course, your mileage savings probably can't compare to driving the same distance in a stock Honda

TTFN

 

[evelrod]

Well big Tom. I just happened to be at the old Riverside International Raceway when SCCA was starting their mini truck race series of the 80's and a couple guys from Ford were out doing some testing. Looking for that "unfair advantage" I suppose. Bottom line---best lap times/top speeds were open bed-tailgait closed. Next was a hard tonneau over the bed. Worst (by a BIG margin) was tailgate removed.
Anyway, SCCA ultimately mandated closed tailgate and no cover anyway.

The problem with testing when you already have made up your mind(right or wrong) is that you sub consiously fudge the test toward the expected results. With that in mind, you might try a few round trips on your truck.

Rod

 

[pennpoint]

bigTomHanks

Just last month I read an article on this topic, in one of the trade mags ("Design News or???).
The conclusion was no real significant gain.
IMHO I dislike seeing the tail gate down, as it looks like the driver forgot and I'm wondering where he/she dropped the load. I'd rather see no tailgate or one of those web belting type or the screened 5th wheel type tailgates.
Ridged tonneau covers look great but if you often load your truck bed with large objects ridged tonneaus are a pain to deal with. Canvas type would simplify that problem somewhat.
Basically I'm siding with "evelrod" here, do some honest testing and see for yourself.

 

[rmw]

In my mind there is a lot of difference between the results obtained in a best laptime/top speed contest and cruising down the slab at 70 MPH trying to conserve fuel.

I'm betting on the tailgate down. I drive a PU when I leave the sedan at home, and once drove it to the west coast and back from the deep south on a regular basis. My experience was that tailgate down (or left home) produced the best mileage. (I ocasionally had cargo that prevented me from using a cover although my daughters PU has one at this time).

Even with respect to the cover, it leaves a rectangular and flat rear end, and testing on the big rigs has shown that the flat back side of a trailer is a loss. One ocassionaly sees rigs with sloped rear end coulings when trailers do not require rear door access to reduce the drag off the rear end.

Not scientific fact, just my $0.02.

rmw

 

[ivymike]

doesn't the back of the cab count as a rect&flat rear end anyway?

 

[ajack1]

Big Tom if you are doing those sorts of miles why not buy something more economical?

I must admit I do find Americans love of pick ups strange as they seem totally useless unless used for what they are designed to do, which judging by the paint work very few are.

I guess paying around $9 a gallon makes you think differently.

 

[GregLocock]

I think the error in the tailgate down theory is that it would only really work if the flow re-attaches to the tray before hitting the tailgate. Why not put some wool streamers on the tray and see which way the air is running ?

My guess is there will be some huge vortices off the trailing edge of the cab roof, and the tray is basically operating in the backwash.





Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[rmw]

It is real easy to see the air flow patterns at the plane normally occupied by a closed tail gate in PU trucks with the net (web belting) mesh often used by PU's pulling fifty wheel campers, when no camper is attached to influence air flow patterns.

That thing, with no more resistance than it has with its copious open area, billows out like a sail in a rearward direction. If you ever get a chance to look at one closely you will have no doubt on the wind forces on the (closed) tailgate.

Ajack1, $9/gal fuel prices would change Americans attitudes about lots of things.....(but let's don't take this thread in that direction-start another one.)

rmw

 

[evelrod]

rmw, the tests I speak of were at all different speeds, including 70...Aerodynamics is not an intuitive science under the best of circumstances. The RIR tests were by Ford Mtr. Co., not exactly your "backyard outfit". I'll still stick by honest testing rather than "intuition". Indeed, aero can be counterintuitive!

ajack1, Why do I drive a PU? Can't say for sure. Perhaps it's my "hillbilly" ancestory. Most here know that I race British cars, mostly (a Mini Cooper and a Lotus Cortina) and have a 40+ year affair with cars British...that in itself gives me reason enough to drive a big Dodge 1T duelly diesel PU just to keep my sanity. Besides it gets 21+mpg and it's paid for!

Greg and Isaac, watching the "tow trucks" run off at the end of an SCTA event at the dry lakes gives a very good view of the airstream. Most run with tailgates up...from experience, I would guess.

Big Tom,---do the tests. Easy stuff. Just don't pre judge the results if you can. Tough to do, sometimes, but worth the effort to "know for yourself". Read my tag line.

Believe only half of what you see and none of what you hear!

Rod

 

[rmw]

evelrod,

I interpreted the information you presented in the earlier post to be more lap time related, which to me is more ultimately power related than fuel efficiency related, hence my response.

Most of my experience of paying any attention to aerodynamic testing related to big rigs, not PU sized vehicles anyway. I was around in the days that Kenworth introduced the "ant eater" cab and was almost laughed off of the globe by a 'stuck in the mud' industry, and now look at over the road trucks/trailers with all their sloped noses, fairings, etc.

And, in the case of my 1t (ford) diesel PU, eventhough I might believe that better mileage can be obtained with the tail gate down, I rarely do it. Looks nerdy to me. Just not the way a self respecting PU ought to look.

Glad to see you posting if you get my drift!!

rmw

 

[evelrod]

rmw, Pats got me pegged. I'm a happy guy with a big mouth.
My shop sign---"Never argue with the Engineer" ---from my kids.

Rod ;-)

 

[TheBlacksmith]

I think Ford did some road speed/MPG testing in the mid-80's. They found that a cover was best, then tailgate up, then tailgate down. Actually a piece of plywood about a foot wide 2/3's of the way back worked well. Explanantion was that the air came down off the cab about there, and if the tailgate was closed, it built up pressure and became a static situation, whereas the tailgate down would relive the pressure and the air flowing along the insides of the bed created turbulence and increased the drag. I use a soft cover and notice a small difference, < 1/2 mpg at 19 mpg, but I like the looks, protection for things from rain/snow and the ease to remove when getting something tall.

 

[frankiee]

My bro use to have a cab on a old chev with 250 and he notices better economy when he had it on.
He experimented some over a couple of years and was lead to beleive that the cap gave better economy
We did not do any accurate testing and most of the driving was short hauls here and there.
I know that a submarine and my bicycle helmet has a sort of bulbous rear for areo reasons. And not just a flat rear

 

[JDana]

My experience with a 1986 S-10 4x4 pick-up showed better fuel economy either with no tailgate or a tall, heavy fiberglass cap. Over a dozen or so 100 mile round trips I gained about 4mpg (about 20%)this way. It took several trips before I believed these results. I really thought the weight of the cap would negate any savings but it didn't work out that way.

For the record, I filled up at the same station before and after each trip. It was right on the corner of the freeway ramp. My destination was also very close to the freeway,<1 mile.

 

[evelrod]

JDana, my memory is, for sure, not what it was, but I do seem to recall the GM high speed runs with a Chevy S-10 at Bonneville a number of years back. Like I said, memory and all, seems like they used a 1/4 shell "camper" add on and hard T cover behind the std. cab with the tailgate up for those 200 mph runs.

I also notice that the NASCAR truck series allows (mandates?) hard tonneau covers with VERY large spoilers to keep the back end of the PUs on the ground.

One more...In the few desert races I have seen, it would appear that the airflow pattern on the trucks (no tailgate) is reversed at the rear for fully half the bed length. Hmmmmm.

Rod

 

[Tmoose]

I'd try short shifting it if manual, and leaving 20 minutes or 1 hour early in order to be able to go steady 60 mph instead of 80 mph or stop-n-go

 

[Tmoose]

Oh, also throw in fresh plogs gapped a little wide, and a can of SeaFoam or Valvoline synthetic fuel cleaner