Improving MPG - PowerStrokeNation : Ford Powerstroke Diesel Forum
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post #1 of 10 (permalink) Old 12-08-2011, 10:12 PM Thread Starter
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Improving MPG

After a couple decades of trying and talking to other guys who have had success at better MPG, I can say that (for pickup trucks) it comes down to three things: Driving (hypermiling), aero, and gearing.

Changing one’s driving style to get better mileage is discussed in many threads, so I’ll just touch it lightly here. Changing your driving style is super cheap (more or less free) and it works to the extent you are willing to make it work. But doing it well requires your attention. You simply cannot text and hypermile.

Aerodynamic drag is the dominant force slowing a truck down at highway speeds (unless you are heavily laden and climbing Sandstone Mountain). Four factors affect aerodynamic drag. First is speed. Aero drag HP goes up with the cube of road speed. The second is frontal drag. Measure all the stuff presented to the wind at the front, especially including the side view mirrors. The third factor is Coefficient of Drag (Cd)– a dimensionless numerical representation of how aerodynamically ‘slick” a vehicle is. For most unmodified pickups this number runs between 0.40 and 0.48. As a point of reference a Corvette has a Cd of 0.28 and a Prius has a Cd of 0.25. Ford brags about their new F150 having a Cd of 0.42. The fourth factor is air density. Air has weight and the truck has to push it aside or over the truck so the truck can move. Heavier air takes more HP to push around. Cold air is denser than warm air and dry air is denser than moist air. Saturated (100% relative humidity) air at 0°F is 16% denser than saturated air at 70°F.

If you took your truck to a race car wind tunnel, they’d work their voodoo and give you a “CdA” number for your truck – expressed in square feet. For an unmodified 4x4 full-sized pickup, one would expect a CdA in the ballpark of 20 square feet. For a C6R Corvette, the CdA is about 12 square feet. No wonder a motivated C6 ‘Vette driver with a LS7 and a 6-speed can cruise at 30+ MPG highway. He isn’t bulldozing much air.

At low speeds, aero drag doesn’t much matter. Below 40 MPH, rolling resistance exceeds aero drag. So if you only drive in the city, forget aero drag. A van is just fine for city driving.

You can’t control the weather so we can forget fretting about air density (except in discussing why winter driving gets less MPG than summer driving). You drive to the traffic conditions, so I won’t say much about modulating speed, but slower is much better. We all hated the 55 MPH limit, but at 55 MPH your aero drag HP is half what it is at 70 MPH. So the guy who can get away with driving like Grandpa will (in the MPG game) always beat the stuffings out of the guy who drives it like he stole it.

Unless you have the bodywork capabilities of a major custom car shop, changing the frontal area of a pickup is a tough proposition. One opportunity might be in the area of side view mirrors. If you have the big TT mirrors, they might represent 15-20% of your frontal area.

The average guy can do something about coefficient of drag.

The big opportunity is the bed. If you can live with a tonneau cover (hard or soft) you can realize a 1.5 MPG improvement by doing that and nothing else. An “aerolid” (a “fastback” version of a truck cap) might gain you 3.0 MPG over an open bed. It did for me. But a flush-top cap will make your MPG worse. You become a van or Excursion with one of those on your truck. Because everybody seems to use their truck a little differently, there is a lot of room for creativity in bed treatments.

Something that definitely works for pickups is wheel well skirts. Most trucks have huge wheel wells and these are in essence mini-parachutes. From the earliest days of automobiles, racers knew that fender skirts cut aero drag. If you go to Bonneville, every vehicle on the salt has fender skirts of some type (and Mooneye well covers).

In general, studying Bonneville racers’ shapes is tapping into decades of aero experimentation in the most ruthless racing of all. They may look weird, but they cut the air as well as it can be cut.

Cleaning up the air flow under the truck is powerful. That double boundary layer under the truck is the draggiest part of all. An air dam helps a bunch, too but you have to be careful you don’t make something that is easily damaged by parking lot curbs and road kill. I made mine of rip-stop conveyor belting. Expensive and difficult to fabricate, but darn near indestructible. A combination of air dam and belly pan gets you into the “Bonneville” class of low air drag under the truck. Unless you are building a vehicle for track use, downforce is to be avoided. These trucks have a half-ton of engine, giving you all the front end downforce you’d ever need.

The front of the truck is not that important, compared to the bed and underbody. Some small cars get a benefit by blocking off all or part of the grille. Might not be a bad idea for cold weather , but pickups are made to carry loads and tow trailers so you’d want to be cautious about blocking off air flow to your radiator. Maybe a variable shutter system like you see on some Ford cars might be an idea. It is usually closed for good aero drag, but if the engine or A/C gets hot, it opens up.

Gearing does not affect road load, but reduces the engine frictional HP and accessory parasitic load at a given road speed. Engine frictional HP is the power necessary to make the engine turn without making any net power to the flywheel. It is somewhat of a misnomer – most engine bearings are plain bearings which have very low mechanical friction. Most of this HP goes to pumping air through the engine. For International and Cummins engines (and most engines in this class, I’d bet) engine frictional HP goes up proportionally with RPM between 1000 and 2000 RPM and increases with the square of RPM above 2000. Looking at engine frictional HP curves I’ve seen for the International 444 engine, my engine need 12 HP less to make itself run at 1325 RPM than at 2000 RPM. That explains the 3.0 MPG improvement I got over a 3.73 R&P.

Gearing is tough to do. This involves changing the ring-and-pinion set to a numerically lower ratio. Most guys go up in ratio to compensate for big tires but lower ratio R&P sets are rare. I have a 3.08 gear set, but I don’t think they are commercially available anymore. 3.55s can be had, though. I’d expect a 1.0 MPG improvement for a truck that had 3.73s (the most common) and somewhere between 1.5 and 2.5 for a truck with 4.10s.
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3.08 gears Gear Vendor Overdrive
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post #2 of 10 (permalink) Old 01-02-2014, 08:04 PM
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Re: Improving MPG

a month ago I was getting about 8 miles per gallon with a lot of black smoke (unburned fuel) and got two of my injectors replaced by dieselogic, apparently injectors were releasing two much fuel volume. smart buy, saved me money in the long run, now im getting 13 mpg. thanks for the other info!!
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post #3 of 10 (permalink) Old 06-02-2014, 05:10 AM
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Re: Improving MPG

This was posted on FTE by nlemerise. It seems to echo everything you say,
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File Type: pdf Cummins - Secrets of Better Fuel Economy Whitepaper.pdf (964.6 KB, 64 views)

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post #4 of 10 (permalink) Old 10-25-2016, 01:15 PM
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Re: Improving MPG

Well written article!

I do have a few things to add.

In the following video the Mythbusters examine the following modifications to a truck's bed to see how it affects the truck's MPG:
Tailgate up (control test)
Tailgate down
Hard top tonneau
Mesh tailgate
No tailgate

Oddly enough, the truck used in the test is Jamie's trusty Dodge Dakota (de-badged... I guess Dodge didn't want to pay for any advertising!)


Here are the results:

WORST: Tailgate down
BEST: Mesh Tailgate (a little more than 5% savings)
All of the rest were relatively the same, and having installed a tonneau recently I can confirm the lack of an MPG boost.


MORAL: If you're buying a tonneau for the sole reason of improving your MPG you're wasting your money. It seems like there's a lot of MPG improvement myths when it comes to vehicles, but this one is especially bad because tonneau-cover MPG improvements seems so intrinsically plausible. The bed just seems like a massive wind catcher!

This also shows that you shouldn't trust any manufacturer's claim on MPG improvement. With the internet, it's trivially easy to verify their claims so spend 15 minutes before spending a few hundred dollars on snake oil. Perfect example for me is the Throttle Body spacer. They supposedly improve MPG, but a quick search on the web told me (for the types of engines in a Dakota) that this really is a big scam.

Oh, and always be skeptical of perfectly nice round numbers! Many tonneaus claim a nice, neat, and perfect 10% improvement.
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post #5 of 10 (permalink) Old 10-25-2016, 01:26 PM
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Re: Improving MPG

having built a bed cover for a truck of mine, i concur. it may have actually dropped the mpg lol


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post #6 of 10 (permalink) Old 03-09-2017, 05:48 PM
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Re: Improving MPG

I'm interested in the difference between the tonneau, and the hard cap; how one increases mileage, and the other decreases, when it would seem that they are essentially the same thing, simply different material.

Can you explain this, please...?

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post #7 of 10 (permalink) Old 03-09-2017, 06:38 PM
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Re: Improving MPG

the tonneau (level with the top of the bed sides) doesn't negatively modify the trailing edge of the overall vehicle so much as a cap (camper shell level with the top of the cab) does, and should help air to flow down over the cab and eventually past the tailgate. Some people like to argue about air circulation in the bed without a cover vs a cover forcing the flow boundary...i've yet to see air tunnel results that conclude the argument specifically for the covers level with the bed (or the absence of). I trust Dave has seen plenty of evidence though, or he wouldn't have posted it in his write-up.

now for the big-ass caps. the cap/topper/camper shell creates a giant low pressure area directly being the vehicle like a bus or van, which is bad for mileage. You can watch simulations of the flow separation on youtube. you want the flow to adhere exactly like its going across a nice smooth airfoil, and the big caps don't allow this (there's no clear edge to follow through with). The tonneau bed cover supposedly facilitates this 'dove-tail' or 'boat-tail' effect, but not to the degree of the 'Aerocap' truck topper.

I've personally not witnessed the advantage from the bed cover, tonneau, whatever its called, but I HAVE witnessed the DISADVANTAGE of the camper shell topper lol


I have no idea if i answered your question lol! I just re-read your question and it looks like there is confusion on what dave meant...


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post #8 of 10 (permalink) Old 03-09-2017, 06:48 PM
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Re: Improving MPG

I think you can re-read this carefully and answer you question...
Quote:
Originally Posted by Dave Whitmer View Post
...The big opportunity is the bed. If you can live with a tonneau cover (hard or soft) you can realize a 1.5 MPG improvement by doing that and nothing else. An “aerolid” (a “fastback” version of a truck cap) might gain you 3.0 MPG over an open bed. It did for me. But a flush-top cap will make your MPG worse. You become a van or Excursion with one of those on your truck....


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post #9 of 10 (permalink) Old 03-10-2017, 12:02 PM
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Re: Improving MPG

Quote:
Originally Posted by Turd-250 View Post
I think you can re-read this carefully and answer you question...
Yeah, came back to say that I didn't need that answered after all; I just got it confused, thinking that the cap and the tonneau were essentially the same. Honestly, I just know it as a shell, but then again, I haven't had one for decades... lol

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post #10 of 10 (permalink) Old 03-10-2017, 01:00 PM
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Re: Improving MPG

Quote:
Originally Posted by signcut View Post
Yeah, came back to say that I didn't need that answered after all; I just got it confused, thinking that the cap and the tonneau were essentially the same. Honestly, I just know it as a shell, but then again, I haven't had one for decades... lol
i get confused all the time! at least until coffee #2 lol
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