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Old 03-03-2011, 05:48 PM   #1
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Default Fuel Economy Improvements

Hello-

I am a new visitor to this forum. I recently bought a 16' Ford Superduty Diesel Box truck that I am using to haul a race car and as a budget RV. Everything about the truck is working well and it's non-turbo 7.3 runs perfectly. The issue is fuel use. I got about 10.5-11.5 mpg on a recent 500 mile trip. I would like to do better.

Right now the truck has no aero improvements at all. The flat front of the box sticks up about 4 feet above the roof of the cab and it has no side skirts either. I am guessing that a fairing over the cab to direct the air over the box could help but how much? Side skirts? What else? It looks like the box is mounted on a 7" spacer. Could that be removed to lower the box for better aerodynamics and easier loading?

As for gearing the engine turns about 2750rpm at 65 mph. Seems ok to me. How much of an improvement do you all think I could get from some basic aero aids like a fairing and skirts? Is there something else I could do that could help economy?

Thanks!
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Old 03-03-2011, 08:32 PM   #2
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Those rpm's seem pretty high to me. I don't know anything about that engine tho. Mine is turning 1450 rpms at 68 mph. That mileage doesn't seem that bad either.
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Old 03-04-2011, 08:12 AM   #3
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I agree with Bob on the RPM. Depending on which model you have, the engine produced 235-275 HP @ 2600 RPM and 425-525 # torque @ 1600 RPM. I would try to gear it for 2100-2300 RPM @ 65 MPH. Should give you the best bang for the $. The fairings will help as well but not as much as the gearing. Aerodynamics have a neglegible effect below 50 MPH but go up rather dramatically above that speed. I would be surprised if you could get it up to 14 MPG. That would be extremely good for what you have. Just my Opinion.
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Old 03-04-2011, 12:56 PM   #4
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Your gearing is similar to mine. I turn 2,500 RPM at about 62 MPH. That is with an automatic transmission though. I know that the manual transmission for my otherwise the same 14" box truck has a much higher top end gear ratio. I have been told that these box trucks are geared for short haul around town stuff, not for down the freeway gas mileage. I get somewhere between 12 and 14 MPG, but that is with a diesel. I also have side skirts, but no fairing above the cab. The top front of the box is nicely rounded though.

I suspect that you and I will both need to do the same thing to get better mileage. Slow down. I try to keep it in the 60 to 62 MPH range.

You didn't say whether you have a manual transmission or an automatic. Being into race cars you may have more experience with changing the rear end gearing than I do. (Any experience would be more experience.) I can't promise how much it would buy you though. Others on this forum know more about that sort of thing than I do.
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Old 03-05-2011, 10:59 AM   #5
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I have a 89 non tubo 7.3 and the non turbo 7.3s never made more than 185 Hp stock.
And that HP number is at like 3100 rpm.
They are gears low for a reason. THey don't pull hills all that well under 2500 rpm. if you hit hills you will lamost wish for even lower gears.
The MPGs you getting sound about right. My rollback that has this engine gets 9mpg loaded or empty but it weighs 11k empty and has 5.13 gears.
I think you are on the right track to try to improve its aero profile.
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Old 03-05-2011, 06:34 PM   #6
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I got a roof fairing for my FL60 w/24' box. my fuel mileage improved about 0.9 mpg. in perfect conditions, I can get up to 13 mpg. A great side effect to the fairing was the stability at high speeds.

I modified a junk yard fairing as to actually save money instead of spending the saving on the fairing itself.


(I guess I need to update my profile picture)
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Old 03-07-2011, 11:33 AM   #7
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Avendlerdp,
Mekanic is correct on the HP numbers. I was looking at the Turbo model. The non-turbo model built from 1988-94 was 185 HP @ 3000 RPM and 360# torque @ 1400 RPM. That probably makes your gearing pretty close. Fairings will help some but do not expect any big increase. You might look into upgrading it with a turbo. Would give you a significant HP boost and, according to the charts, they do better on fuel. Not sure how difficult that would be as I have never owned one. Just a thought. MMM
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Old 03-07-2011, 10:51 PM   #8
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you may be able to lower the box quite a bit if you don't mind the tire humps on the floor.

the truck yards are full of old fairings so after lowering find some thing that fits.

I like my air-tabs, may get a bit better milage, but the buffeting and side wind difference is dramatic.

hang some big light weight boxes under your box, then sheet the bottom axle to axle to clean up the airflow under the truck.

There are other ideas, but you really have to run the dollars spent over potential savings.

does it make financial sense to spend 1000 dollars on a gear change for a 1/2 mpg improvement when you only drive a few thousand miles a year?

My project started out as a mpg project, I get 10 at best with ULSD and I'm at the point that the easy stuff is done and to change out anything else doesn't math out even with 4 dollar jelly.
-blizz
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Old 03-08-2011, 10:52 PM   #9
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Hey Blizz-

Your post reminded me of those tabs I noticed following your build. Looks expensive. Did you notice enough to be worth it? I will be running a toter/gooseneck rig, is that something to use on both, or just the back of the trailer? I've not really seen those mentioned anywhere else except for your build. Great job by the way, your pics are my training manual.

Dave
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