Gas mileage drain?

[chesspirate]

It wasn't a big deal untill gas reached $2.60+ out here but...
For the longest time my 1991 4.3 TBI averaged around 20 to 21mpg's mixed use (i drive like a granny). But lately i've been anywhere from 15.5-18 mpg's mixed use. Between then and now I've installed a hi po straight through muffler that seems to be one culprit and i've put in a plastic storage box in the bed. But, i've also removed the tailgate, bumber, hitch, spare tire, & bed rails. wich in my head balances out the weight issue. I've also installed an MSD 6A between then and now and that did nothing (i'm actually suspicious that the 6A is doing to good of a job and fooling the O2 sensor thus dumping more fuel into the mix) who knows? Besides all that other stuff, regular maintenence has been used, same gas has been used, pretty similar driving conditions etc. What other possibilities are there for me to regain my Mileage?

[blazen]

drive with the tailgate down. that helps with wind resistance. and also keep the windows closed that will help alittle but both these things only really work at highway speeds. and yes gas prices suck but 2.60 has to be the good stuff because the cheap stuff around here is 2.15.

[chesspirate]

NOPE! 2.60 for reg. Welcome to southern california. We passed 2.15 about three months ago. Also, in my post you can see that the tailgate is removed.

[blazen]

oh sorry when i read it i thought it said tail light was removed and i was like what the....why would he remove the tail light and what does it have to do with gas milage. i guess i should read more carefully next time.......sorry

[OBI WAN]

OK first off, until May hits the US gas reserves contain oxygenated fuels which will drop milage off 2-3 mpg or more. Once May hits this is then removed from the market.

2nd how many miles? 02 sensors last 80-100K before they begin to die off reading and will run it richer than before.

Muffler?? What kind is it?

Tailgate killed you more than it relieves windage problems. Air behind a straight cut back like a truck rises from the ground and pushes the vehicle along. Ever wonder why Pro Street Trucks have spoilers that hang of the ass>> Down Force. After a rain storm, snow , whatever look at the rear end! See how messy it is versus the rest of the vehicle, its the air rising and pushing it along!

[bvr775]

you can get or make a bracket that goes on the gate to hold it partaily open at about 45* that is shown to to give a 1/2 to 1 mpg gain. I've also seen a strut that gose on the gate that you hit a switch and it automaticly open the gate to right angle. Tailgate down will actually lose you a 1mpg verses closed gate. That was proven many years ago and almost put airgate out of buisness.

what you really want is a light weight cover for your bed.

[chesspirate]

Well, the O2 sensor was replace approximately 30k miles ago, so i don't really think that thats it, but in the back of my mind I wonder.
The muffler is a bullet muffler semi-straight through. If you look at it from inlet to outlet it looks like somebody stretched a Z out.

Somebody has got to explain the wind pushing the tailgate to me, i'm baffled, I don't see how at 55+ MPH that wind traveling under the vehicle comes back to 'push' the vehicle.

[Matt]

The sharp edges of the truck make a break point for air turbulence, much like dimples on a golf ball 'break' the smooth lines of the air. Suprisingly, given certain forms of turbulence in the right areas (again think golf ball), wind resistance goes down. The air that would have gone straight by the truck actually swirls around 180 degrees and pushes a little bit on the truck. Youll see this same princliple in play in just about any new car. Look at the trunk lid. There usually is a small rounded or sharp lip that can make it look sporty, but is actually there to cause the right kind of turbulence.

[chesspirate]

So, I should reinstall the tailgate? and maybee find a way to keep it open at 45%?
Okay, I'll give it a shot, that means I've got to put the bumber back on so the license plate will have somewhere to go again. bummer.
On that wind thing again though, does anybody have test results from a wind tunnel that the factory tested at way back when or is there some way of getting that info, does anybody know the engineers that design vehicles for a living or know someone that knows someone.
My problem is this, with the Honda Insight, wich is the most fuel efficient vehicle out right now (hybrid) the but is very small, and will solar racers, the tail sometimes comes to a point, and they say its to reduce drag and wind resistance. So if a big but helped 'push' a vehicle along wouldn't the most efficient vehicle designs incorporate that?

[Matt]

Wind resistance will decrease when the object comes to a point like that much like an airplane wing, but that design is largely impractical for any automotive use as it has to taper out roughly 2/3s of its length for that effect.
And also if you notice even the honda insight has that same 'break' (the little spoiler, lip thing) that I was talking about earlier on the trunklid. Its drastic how much of a difference creating turbulence can make on something thats not as aerodynamically efficient as an airplane wing.
http://automobiles.honda.com/models/exterior_gallery.asp?ModelName=Insight

[DeathtoRice]

ford did a test on an F150 and found a negligible increase in fuel economy with it up. Something about a negative pressure area inside the bed with it up, helps the air flow smooth over it. like an invisible tonneau cover. But the best way is to get a tonneau cover. they tested it with one of them and mileage increased slightly vs gate up.

[Matt]

Oh yeah, there are some writings about this, but youre not really going to find them online as they go against conventional wisdom so the normal joe wont think anything of it. Heres one I found, but again, its not available in entirety online.
http://www.sae.org/servlets/productDetail?PROD_TYP=PAPER&PROD_CD=2004-01-1146

[Matt]

Heres an automatic tailgate opener that opens to certain angles at certain speeds. Neat idea, but seems a like it could be expensive and a too much of a hassle for the smaller mileage gains.
http://www.droptail.com/index.htm

All this talk of tailgate down, tailgate up is good fyi, but Im pretty sure that its not causing your mileage loss 100%.