dplummer87

Just wanted to chime in on the mythbusters statement. Our 06 dodge mega cab cummings gets 20.5 with the tail gate down and online 19 with it up. The truck they used in mythbusters was some run down POS. I didnt like that episode. didnt seem logical because in my opinion, the vehicle used was not a TRUCK as most would call it.

nagata

Well, on the latest tank, I mixed in a hundred or so miles of 50-60 MPH driving (cruises just above 2000 RPM at 60 MPH, the other ~200 miles was mostly 70 MPH highway and some city) and that bumped the mileage up to 16 MPG. So, apparently when you run it at 2500, it sucks down significantly more fuel. 500 RPM doesn't sound like very much of an increase, but obviously, it is in terms of fuel usage.

So, I'm undoubtedly out of the optimal efficiency range, and the only reasonable way I'm gonna get it back down is to drive slower (not gonna happen, I'm in construction and waste way too much time on the road as it is) or scale back the rear gearing.

Any way I can accurately figure out what sort of RPM I would be running with 3.73 gears? 3.55 gears? Anyone have a 4.2L V6 5-speed with 3.55 gears in the rear that can let me know what RPM it sits at when cruising at 70 MPH, and what sort of gas mileage you get? Might have to start a new thread...

dplummer87

Honeslty, I would probably go with the 3.55 gears.

My 08 FX4 (Screw) Tow package has 3:73 gears stock 18" rims/tires
Cousins 08 FX2 (Screw) 3:55 gears stock 20" rims/tires beats me everytime. Even with a performance chip and CAI in mine. So if you want a little more speed, but not sacrifice MPGs, go with the 3:55.

nagata

Thanks. Given the increased tire/rim combo from just under 30" to 32", 3.55 gears would effectively cancel out the size increase and get me back to a close-to-stock "ratio", even though I'd have 3.55 gears instead of 3.31 gears. If I went with 3.73 gears with the bigger wheels, the end result should be close to the same result as if I had installed 3.55 gears with the stock wheels. Hmmmm, kind of confusing...

Mod (Ret.)

Not sure I understand your statement "...was not a TRUCK as most would call it." It was a pickup, it had a bed. The fact that it appeared to be a daily driver, used for hauling, and did not have as nice of a paint job as some that are posted here, well, alright then; I guess some might refer to that as a POS, but there's plenty of those around, being used for what they were designed for; work vehicles. In any event, they ran the truck, using several gallons of gas with nothing removed, nothing installed, to get a base MPG indication of that truck. Then using the same truck,they performed the tests as described. No increases in MPG.

I won't doubt you that you do see gains in the MPG catagory when your tailgate is down, I will however doubt the so called "improved MPG" statements from the tonneau cover manufacturers.

I like the looks and protection/security that some tonneau covers can provide. I'd even consider installing one on my truck some day (I have other priorities that my limited spending budget allows at the moment). I'm just commenting on the advertisement that some aftermarket companies use to sell a product, with no actual proof of their statements. Seems everything we buy for a vehicle today has a "saves gas" label on it. Where's the proof?

reginhild

Installing a tonneau can help improve MPG.

I always take Mythbusters episodes with a grain of salt. How many different model and configuration types did they test? What was the weight of the tonneau used.... Mythbusters episode reference: http://kwc.org/mythbusters/2006/12/e..._revisite.html

Their results actually got the same MPG with tailgate up compared to with tonneau cover. Now account for the weight of the tonneau cover being about 100 lbs and you actually had better aerodynamics but that was countered by the weight of the cover. Go with a lighter tonneau like lazerlite aluminum (48 lbs) or an ABS cover (60 lbs) and you will not have so much extra weight to haul around and should get better gas mileage.

An example of a good SCIENTIFIC test was done by the Society of Automotive and Aerospace Engineers (SAE): http://ecow.engr.wisc.edu/cgi-bin/ge...hi1/pickup.pdf

Their testing shows that other variables make a difference such as crosswind and bed length.

For their testing of pre-2004 model F-150s they found:

1. In most cases a partial box cover (at the rear) or full tonneau gave the best aerodynamics.

2. For the 8' bed standard cab trucks the tailgate down but not removed gave better aerodynamics than with tailgate up. Tailgate off gave better or worse depending on how much crosswind compared to speed. Full tonneau gave the least drag in all conditions (best for MPG). Mesh gate gave the highest drag in all conditions (worst for MPG).

3. For the 6.5' bed super/crew cab trucks the tailgate up gave better aerodynamics than down or off (like Mythbuster results but opposite of 8' bed results). Tonneau covers gave the least drag in all conditions with a partial cover slightly better than a full cover (best for MPG). Tailgate off gave the highest drag (worst MPG).

If you look at the flow models you can see that the difference in Cd occurs after the cab, so you could extrapolate that the bed length difference will be the decider in what method you should go with. The GMC and Toyota tested also support this conclusion with their 6.5' bed trucks having similar results to the Ford 6.5' beds. (Tailgate up is better than tailgate down)

In all cases, a tonneau lowers drag...but you have to account for the weight of the tonneau possibly having a negative effect on MPG. Lesson: if you want improve aerodynamics and lower MPG with a tonneau, pick the lightest available.

Here is more scientific testing by SEMA showing that tonneaus improve aerodynamics: http://www.trucktestdigest.com/PDFfi...uStudy2007.pdf
Summary of some of their results: Hard tonneaus did better than soft in general with 6.5% or better drag reduction compared to 5% drag reduction at highway speeds. Reduced drag primarily occurs at above 40 mph and the benefit increases up to 65 mph where it levels off. They also mention that the weight of heavy fiberglass covers can negate benefits.

Will repost this in new dedicated thread.

ford4wd08

uh bud i would check your facts ive seen that episode and i believe that they were new f-150's, so check it out im positive they were 04 and up f-150's, or at least the episode i watched when they checked MPG with tailgate up and down and the fishnet thing

reginhild

The first episode (tailgate up vs. down) looked like a F-150. The second episode (tailgate up vs. down vs. off vs. meshgate vs. tonneau) looked like an older Dodge. Youtube video: http://www.youtube.com/watch?v=r3aqHbD-O9E

Go to the Mythbusters site and there is a comment about their method of using a fuel flow meter. They did not account for fuel backflow to the fuel tank so their readings are not good.

reginhild

P.S.: started a new thread dedicated to Tonneau Covers and effect on Gas Mileage here: https://www.f150forum.com/f2/tonneau-covers-gas-mileage-20851/

New thread has more links to testing showing that Tonneaus do improve MPG (in addition to the links above).

jacobs4x4

Something my granddad always told me was every 100rpm over 2000 = 1mpg decrease... Thats probably why my truck gets 10mpg and because its a 97 but I have 35's with a 5.4 v8 and 3:73 gears sometimes I dont shift till 3000rpms...