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Old 04-23-2013, 03:30 PM
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MPG question

I recently picked up a Ď99 dually (mfg date 6/98) just outside Dallas with 189k miles. Looks to be all original - 235/85-16 tires, auto trans, 4.10 gears, filter and exhaust. Set the cruse at 75 and drove it to Houston. On the trip I got 16.93 mpg. The truck is to be used primarily for towing our boat (12K lbs) around as the '06 F-250 4x4 daily driver feels as though it is working at its limit when I pull the boat with it. So to help out with the air flow through the motor I replaced the exhaust with a Magnaflow 4" and the filter with an AFE system. The first trip out I checked the mileage and it had fallen to 15.5 MPG. Figuring that my math calculating abilities were at fault I checked again after another tank and it was the same. I am not a heavy foot driver, I use the cruse a lot thinking that helps with the mileage. I asked around about the decrease and everyone is surprised that it has fallen. One guy said that he thinks itís the EGR because there is not as much back pressure and it is telling the computer to send in more fuel. Another guy said that I just need to chip it and that should solve my problem. So, I am at a loss and thought that I would ask the think tank for help.
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Old 04-25-2013, 02:52 PM
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So today I got another theory as to the decrease in my fuel mileage. This guy said that the loss of back pressure is indeed my problem and said that I should reinstall the cat. He said that the 7.3 was not designed to have that much free flowing exhaust. Also said that I could keep it like it is but that I would have to increase the size of the intercooler and associated pipes, hoses etc. Any thoughts?
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Old 04-25-2013, 03:52 PM
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Setting the cruise control in these trucks will not help with mileage. They tend to over compensate with power aka fuel to keep your set speed (especially at 75 I'm sure your RPMs were pretty tall while towing). Forget the cruise control and use your own judgement to keep the speed up and the RPMs down. 7.3s dont have an EGR.

Im not sure where your friend is getting the facts. The turbo is enough back pressure for the engine and putting a cat back on would be unnecessary restriction. Theres no need to get a bigger intercooler unless youre getting the whole package ie fuel, turbo, injectors, etc.

Bottom line is to keep the foot out of it and take it for what it is--a diesel dinosaur that isnt built for economy but will pull what you need.

Congrats on your rig

Last edited by AgTires4295; 04-25-2013 at 03:56 PM.
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Old 04-25-2013, 04:35 PM
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I agree with the above poster. Reduced back pressure is a good thing in turbocharged engines.

One thing I've noticed with my truck is that it is very hard to calculate fuel mileage due to the foaming of the fuel. Unless you can guarantee that you are filling the tank to the exact same point every time, you won't ever be able to get an accurate number. I, personally, fill my tank very slowly to get it as full as possible after the pump initially clicks off. If I just go by the nozzle shutting off, I noticed that the amount of fuel I can add varies greatly from station to station.

There is a popular mod where you can cut the vent line in the tank. You can check it out here: http://www.guzzle7pt3.com/harpoon.php

I would imagine that it could help alleviate some of the issues with foaming causing the nozzle to shut off early; however I cannot comment, since I haven't felt like dropping the tank yet to do this (or the Harpoon Mod).
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Old 04-25-2013, 07:38 PM
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I have the same setup as you powertrain wise, 99.5 f350 drw 4.10s and 235/85r16's I usually go around 60 on the freeway towing or not towing just to keep the rpms down. I get around 15.1-16.0 thats with at least a 10 minute warm up between driving and a decent amount of idling when Im in parking lots ( I keep the truck on when I do short run ins to business') never really tried just starting and letting it warm up for a minute and then roll out. probably get a mpg or two better if I did
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Old 04-26-2013, 05:50 AM
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i really think your sweating over nothing IMO
Quote:
Originally Posted by Drew Bob View Post
On the trip I got 16.93 mpg.

thats pretty damn good , esp for a 4.10 geared dually

The first trip out I checked the mileage and it had fallen to 15.5 MPG.

a slight grade where you filled and maybe squeezing an extra gallon in than before could easily make a 2mpg difference


One guy said that he thinks itís the EGR because there is not as much back pressure and it is telling the computer to send in more fuel.

I probably wouldn't ask him for advice anymore

Another guy said that I just need to chip it and that should solve my problem.

Will add power but dont spend the money expecting mpg's or you'll be dissapionted
.
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Old 04-26-2013, 07:06 PM
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Ok so I have taken in the comments and have thought about the different trips to get my mileage numbers. The trip from Dallas was about 290 miles. It was a single stop trip, we ran after 9:00pm, it was cool that night, I did not run the a/c and I donít know if we had any wind. The other trips have been round trips to Matagorda with one or two stops each time, we go through two small towns with a bunch of lights. We also have a couple of wide spots in the road that we have to slow down to 55. The wind has been really blowing lately but that is usually only affecting me one way. Round trip is + or - 200 miles, it depends how much putting around town we do. So taking that into consideration I guess that I will not be able to tell if my mileage really fell or not until I duplicate the same trip.

Other things: I never thought about the foam building up in the tank while I am filling up, I try and park level each time I fill up, I will probably still use the cruse cause I am a little lazy and I find myself always trying to be the leader of the pack if I donít use it.

So I guess that I will just be happy where I am at for the moment. I will be towing the boat back home this weekend keeping the rpmís right around 2000 and then check the mileage. Iíll let you know what I get. Thanks for the help.
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Old 04-26-2013, 09:27 PM
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Sounds like a good plan! Considering you've got a dually with 4.10 gears, I'd say 15.5 mpg is great mileage, especially at 75 mph!

When I do calculate my mileage, I find that I'm somewhere around 15-16 mpg (I've done better and I've done worse, but this about the average). I have a SRW 4X4 with 3.73 gears and 35" tires, and I typically drive around 70-75 on the highway, FWIW.

Considering these trucks only have 4 speed autos, their fuel mileage always amazes me. I had a 2005 Duramax and typically only got 15 mpg with a 5 speed auto.
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