actually, there have been studies done, and it turns out that it makes little or no difference.
the "swirl" that typically occurs in an empty pickup bed actually forms a bubble, and the air coming up over the cab (where most of the drag occurs" slips over the top of this bubble.
when you open up the tailgate, it disturbs or eliminates this bubble of air, and actually causes more drag than if you leave the tailgate closed.
from my own experience, with my last truck, I had a tonneau for a while, and a cap for a while...drove around with the tailgate open a few times (meaning: I filled the tank, recorded the mileage, opened the tailgate and left it open until it was time to re-fill the tank...then actually DID the math). nothing made any difference. That truck got 15mpg, rain or shine. I usually kept track of it, and when the mileage went down, it was because maintenance was needed. the ONLY time I ever saw a significant gain was one extended vacation that I took, which involved driving a few hundred miles on secondary roads, where the speeds were fairly steady and 45-55mph. The mileage jumped up to 19 for that tankfull. I was shocked, and never saw that kind of mileage again.
The shop down the street continually has diesel powered pickups in there being upgraded for more horsepower and at the smae time gains 2-3 mpg.
After talking with Tim (manager of Tom & Gigs) here in Coos Bay. Here is what he told me. Keeping the diesel engine breathing effieiecntly is very important. Before they do any engine modifications they change the air intake system to something that will increase airflow into the engine and change the exhaust system to to decrease the exhaust temps.
After that there are other tricks to better gain HP and fuel economy but I didn't get it all written down.
Their shop rig is an 04 Dodge Cummings diesel that has been upgraded and it sounds very quiet (except when you push it) has around 70hp over stock and runs around 23 mpg while crusing the highway.
I guess there are not alot of places that do this kind of work so people from all over travel to Coos Bay to have this done so they are very busy.
Hope you don't mind me asking the pros down the street about ways other than tire pressure and general maintenance of the vehicle.
Do you have the LB7 or LLY Duramax motor? I ahve heard good things about the tuning programs from www.westerscustomtuning.com. Especially good gains in mpg with the 8.1 motors. Check them out and give them a call, they'll be able to work out with you on specifics of a tune to give you max mpg, and then will figure out your price.
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Tow Rig: 2005 GMC Sierra Denali AWD, 6.0L HO- LOTS OF UPGRADES to help Towing (and Fun)
Trailer: 1968 Tradewind 24', Land Yacht trim (Shell-Off Rebuild and upgrades)
***Missing from my life*** WBCCI # 13657
++Take a look at my restoration photo album++ http://picasaweb.google.com/ChadsAirstream
actually, there have been studies done, and it turns out that it makes little or no difference.
the "swirl" that typically occurs in an empty pickup bed actually forms a bubble, and the air coming up over the cab (where most of the drag occurs" slips over the top of this bubble.
when you open up the tailgate, it disturbs or eliminates this bubble of air, and actually causes more drag than if you leave the tailgate closed.
I did similar testing (got out pen and paper, and did the math) like you did. My mileage went up .2 mpg with a tonneu, and tailgate closed (of course). It held nearly steady with a topper (my theory is the extra weight of the topper negated the fuel savings of aerodynamics), maybe .1 mpg gain, if that, and lost .1 mpg with the tailgate removed, and no topper or tonneu cover. The plus of the cover or topper is you can haul stuff without it blowing away, or getting wet, and less chance of it being stolen, with a (very) small increase in MPG. However, we are talking MPG, not security this time around.
Keeping your vehicle properly maintained and tuned, and adjusting driving habits, seem to render the best results.
Hope you don't mind me asking the pros down the street about ways other than tire pressure and general maintenance of the vehicle.
Darryl
By all means, ask away, just don't keep the info a secret, we all would like to know.
There are things you can do, like reprogramming the ECM, to improve horsepower and milage, but that can create a problem with warranties, and possibly emmissions tests, so I didn't want to bring it up, as advice to do.
The F350 owner I mentioned above is real handy with cash, so was able to buy nearly every accessory that can add extra power. We don't have emmissions testing here (yet) so he can get away with his mods for now.
I did similar testing (got out pen and paper, and did the math) like you did. My mileage went up .2 mpg with a tonneu, and tailgate closed (of course). It held nearly steady with a topper (my theory is the extra weight of the topper negated the fuel savings of aerodynamics), maybe .1 mpg gain, if that, and lost .1 mpg with the tailgate removed, and no topper or tonneu cover. The plus of the cover or topper is you can haul stuff without it blowing away, or getting wet, and less chance of it being stolen, with a (very) small increase in MPG. However, we are talking MPG, not security this time around.
Keeping your vehicle properly maintained and tuned, and adjusting driving habits, seem to render the best results.
Consumer Reports did this a few years back...far enough back that I can't find it in the bathroom stack
and came to the same basic conclusions. We currently use a soft tonneau cover on ours...I would prefer the cab height shell...but the truck belongs to my wife
Aaron
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....so many Airstreams....so little time...
WBCCI #2449 AIR #2495
Why are we in this basket...and where are we going
Thanks, all, for responding. We have the soft tonneau cover and I have reduced mph, so aside from spending beaucoups on add-on modifications which would seem to negate any savings or parking the truck, it looks like we are as good as it's going to get. I'll just hope that with more breaking in, the gas mileage number will improve, as Norby suggests. I just wanted to check with those in the know Thanks again. I hope this was interesting to several of the forum readers who may have been wondering the same thing.
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maxandgeorgia
1995 Airstream Classic Limited 30' ~ Gypsy
Chev Silverado 2500HD Duramax/Allison, 4X4, Crew Cab
WBCCI #5013 AIR #2908
WDCU
Go, Mizzou...Tigers on the prowl!
I gained about 4mpg in my car by removing the roof rack. It was a "safari" style basket, fairly low profile with bike mounts and a fairing. All in all, I could add another round trip on my daily commute before I had to fill the tank again. It probably helps that my AC is busted. I saw an episode of Mythbusters where they ran an experiment of AC v. windows down. Windows down won the miles per gallon competition, though I think they had the AC running full blast.
I've tried most everything and my 460, with or without the topper gets 10-11 empty and 8-9 towing. I think most of the products that claim better gas mileage are in the same categories as wrinkle creams and weight loss pills.
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Paul & Annie
"No matter where you go, there you are..."
WBCCI #7162
Charter Member - Heart of Texas Camping Unit
AIR #1565
Windows down won the miles per gallon competition, though I think they had the AC running full blast.
- Jill
Jill, most car's air conditioning compressors (the part that robs gas mileage) operate all the time the switch is in the "on" position. The only difference when you move the thermostat is the controller allows hot water from the engine to enter the air blend area to make the temp warmer. So, honestly, it didn't matter if they ran the A/C full blast or not, it would generate the same drag on the engine, and the parkas were great for effect.
Being new to the AS hobby I haven't had too much experience with pulling/towing trailers. We made our first trip last weekend to a state park in MN which was about 100 miles from home. I topped off the diesel in the mighty Duramax (lly) just before lift off and reset the trip odmeter and personal MPG on the computer. At the end of the trip I computed the MPG myself to see if the personal MPG reading was accurate and guess what - they matched showing 12.6 MPG traveling on the interstate at 67 MPH! So the truck's computer seems to be accurate. Tomorrow I'm taking the back roads and will be cruising at the posted 55 MPH for a 150 mile round trip. I expect the mileage will be in the 14 range. Just havin' fun! Dale M
I'm sure the gas prices will drop after Labor Day, but it sure has captured attention now!
Quote:
Originally Posted by darryl97420
The shop down the street ...(snip) ........breathing efficiently is very important. Before they do any engine modifications they change the air intake system to something that will increase airflow into the engine and change the exhaust system to to decrease the exhaust temps.
Darryl
In talking to many mechanics I hear "keeping the engine breathing well" is one of the most important tasks.
I have heard that the K&N air filters are only better at wide open throttle (which I never do) and that the Banks exhaust systems are excellent (if you have the cash) for lowering pressures and increasing flow. One mechanic recently advocated the Tornado (which acts like a mini turbine swirling the intake air), but I saw very cool responses on a Google search.
So what moderate cost changes really help the engine breath easier? I know - change the air filter. What else?
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