I'm curious why you think that the 2-3MPG increase is BS based on only your experience? I monitor my Edge and can see that my steady state fuel consumption (instant fuel economy reading) is at 20-21MPG with my tonneau. Before the tonneau I was getting 15-16MPG at the exact same driving conditions...within 10 RPM on the tachometer. I can go out and repeat the results every time with or without the tonneau cover on.

 

Because if I didn't see anything and I truly believe they do little to no difference, why would someone else get 3 mpg more from virtually the same thing and I got nothing? If you picked up 3 mpg then I'm guessing more of that is from driving habits than by simply throwing on a tonneau cover, especially since Mythbusters said they don't do a damn thing either. I just laugh anymore at these gas mileage threads and the claims that some people make. Everyone on F-150 forum claims to have picked up 3 mpg from something.

F-150 online gas mileage increases:
Intake = 3 mpg
Exhaust = 1 mpg
Tonneau cover = 3 mpg
Gas mileage tune = 3 mpg

lol

So let's just pretend based on the theory.....I currently get 16 mpg at 80mph. According to my chart if I add the right mods I should be getting 26mpg @ 80. Now if I just had a tail wind I'd probably get 28!!! It's almost a Honda!

Still wonder why I don't believe it?

And monitoring "instant fuel" is hardly an accurate way to record mileage. Come back to me when you've written down by hand calculations after 20 tanks with both the tonneau open and closed driving the same roads at the same speed and tell me you gained 3 mpg. I have a feeling you won't be coming back with that claim.

 

My improvement definitely isn't due to my driving habits. I can monitor engine speed, vehicle speed and fuel consumption with my Edge. My habits for the purpose of gathering data are the same. Scientifically speaking, my results are repeatable time and time again. I'm looking at the raw data (and I can't figure out why the instant fuel reading isn't valid in your view) and it shows me that I'm getting better fuel economy with the tonneau.

By the way, I'm a mechanical engineer, spent 6 years in the auto industry as a powertrain engineer...at Ford. I'm simply letting the data drive my decision. And yes, I do have the pre-tonneau and post-tonneau data in hand to back up my thoughts and opinion so I'm coming back to you with the same statement that I made earlier.

Additionally, windtunnel testing that has been performed (I have observed the test and witnessed the results) with a smoke wand show that the low pressure turbulent flow behind the cab of the truck is desireable to a point. The air flow separates at the front of the truck and "swirls" just behind the cab. The swirl that is present with a tonneau creates a "push" to help propel the truck. Near the rear of the bed (with a tonneau) the airflow reforms but separates again after the tailgate and forms again after the tailgate and gives another nudge to the vehicle.

Without a tonneau the low pressure "swirl" becomes so large and creates such a bubble inside the bed of the truck that it inhibits the airflow from forming after the tailgate and actually creates turbulence behind the truck. The turbulence actually results in drag and slows the vehicle (slightly). The drag value starts to increase exponentially after around 60 MPH.

Your thoughts on the theoretical gain at 80...the wind drag for an F-150 is tremendous. Any gains that you would be getting through mods are going to be completely nullified at that speed due to the sheer frontal area and drag the truck has. That is just physics.

Now, take those mods and drive at, say, 55 MPH (a point that the truck will experience a significantly lower amount of drag) and you should be seeing a fairly significant improvement. After about 60 MPH the drag coefficient drives the top speed limit on a truck. An improved intake and exhaust allows the engine to breath better subsequently improving not only fuel economy but power as well for speeds less than 60 MPH. More available power at a constant speed will result in lower fuel consumption.

As far as Mythbusters...I'm a big fan and have watched a number of shows. They don't use an Edge monitor but still use a flow meter hooked into the fuel line. If you are taking them at their word then your comment above about dismissing instant fuel consumption will invalidate their results. There really isn't a difference between monitoring an Edge (like I do) and the Mythbusters flow meter. Now you have to pick a side of the fence and stay on it regarding the monitoring of fuel consumption.

 

Bdholsin,


You present some very good information in your response. Something I didn't take into consideration and should have is vehicle speed. Driving as slow as you say you do I can see where you would gain some mpg gains but I still question 3 mpg. If you can't get 20 mpg at 60mph with no tonneau and 3.55 gears that seems pretty bad. I never drive slow enough to test any kind of mpg gain like that. Most highway speeds in Florida are 70 so I'm typically doing 80 where I'm not going to see any kind of improvement. If I'm only do say 55 or 60 it's on a side road and I'm not going to stay at that speed long enough to test out any gas theories.

You say your driving doesn't have anything to do with the gains but I would say they do. If you drove faster all the time you wouldn't see 3 mpg gains just like you said. lol

So I guess when people say they saw 2-3mpg gains with different modifications that is a red flag for I drive slow because anyone who is driving 70 or more on the highway is not going to see significant mpg jumps and you just confirmed that. Drag is just too much on the highway for these trucks.

From now on when I read posts where people say I get such and such on the highway I need to make sure to find out at what speeds they are driving at. Just as you said, that has more to do with fuel economy than anything else. Some interstates the speed limit is 55mph, whereas where I live they are more like 65-70. 55-70 is a big fuel economy difference for sure.

 

Just a little info:

Before my tonneau cover was installed: 17 city/21 hwy
After my tonneau cover was installed: 18 city/23 hwy

I did the conversion a couple of times, and I was coming up with the numbers (okay, I was getting 22.66 hwy-might as well round it off)

No BS here. While the weight factor is an issue, my tonneau cover weighs in at about 80 lbs, while a full size topper weighs approximatly 150-200 lbs, depending on the options you get (windows and such). I can see no difference with a full size topper on, because of the weight, but with a tonneau, not so much.

 

Sorry...I meant with a tonneau, a marginal improvement in gas economy....my bad!

 

Thank you for the generous response. You are certainly a gentleman for acknowledging my point and not getting defensive from my previous post. I'll admit I was a bit agitated and was more focused on proving myself as "right". What set me off was when you said that the 2-3 MPH improvement was BS gave me the feeling that I was being called a liar. Looking back I think that perhaps you weren't trying to make that statement. I just took it too personally. I'm not being sarcastic...I'm extending an olive branch in peace if you took my previous comments as forceful or as an assault on you. As a matter of fact I think I need to step back and apologize if I came off as a jerk. I'm still pretty passionate about cars/trucks (from working in the auto industry) as well as my truck. With that being said I'll step down from my soap box and try to be a little more personable.

Here in Michigan, as in Florida, the speed limit on our highways is 70 MPH as well. However, I guess what I didn't convey very well that right here in my little corner of the world we have a bunch of construction on the stretch of highway that I frequently use. The posted speed limit in the construction zone is 60 MPH and 45 MPH where workers are present. So for the most part I'm not exceeding 60 MPH (and subsequently not sending the drag coefficient that I mentioned earlier resulting in lower fuel economy). As a matter of fact, due to the construction on the highway I wound up clipping my passenger side mirror on an overpass support column (the road currently butts right up to it). Hence, I try to avoid the highway construction as much as possible and wind up using the standard business road with a posted speed limit of 55 MPH. This is where I'm really getting better fuel economy.

I think a safe statement is that mods that are made for fuel economy improvements are valid up to about 60 MPH (or close to that speed). After that point more power is nice but probably isn't going to do much for your fuel consumption. That isn't to say there aren't things that can help but gains will probably be minimal at best.

Also, I try to avoid city driving as much as possible. The stop and go cycle is killer for fuel consumption. I make it a point to drive the posted speed limit when I'm heading through town as if I can hit the first light at green and do the posted limit, I can hit about 6 or 7 lights green. That certainly helps but I still see my average mileage drop when I get into Midland.

This brings me to another and my final observation. We seem to have gone off on a major tangent from the original thread subject so I'll try and bring us back to the original discussion.

Bottom line, if you want to save fuel, drive slower. I'm currently artificially forced to do this but the results are working in my favor and my driving habits have changed. I'm certainly aware of this and in the future, with the current gas prices, I'll probably maintain my habit of avoiding the highway for the most part. If it takes me an extra 5 minutes to get to town (it is only about 10 miles away) then fine, I'll look at it as an extra few minutes I get to spend in my truck and listen to may Sirius satellite radio.

 

Haha, don't worry I did the same thing. I guess what bugs me is that more often than not (and this isn't just applying to tonneau covers) but you'll see people say "I gained 3mpg with an new tune", or intake, whatever but they never mention the fact that they rarely exceed 60, don't travel the highway, do this or do that. They make the statement very generalized like it's normal to see 3mpg across the board. They don't mention the fact that in order to do so you can't go over 65. So you get some other guy who gets a tune, cover...goes out on I-95 going 80mph wondering why he's not getting better fuel economy. You know? It's one thing if you get better results but people need to explain why. I don't know why that bugs me so much. lol

 

I get 17.6 driving like a bat out of hell, with jlt and straight pipe(probably the most free flowing setup, also have Efans, throttle body, custom tune for performance