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How to get better fuel economy w/your diesel



  • Just bought an 06 2500 SLT diesel 4x4 4 speed auto last night. When is it ok to change to a cold air intake and put the cat back system on it? I know those two items won't void the Cummins warranty, but is this something that can be changed immediately or should I break the motor in first? As for fuel additives I know Cummins is VERY particular about their motors, is that allowed or not? Lastly when does "full break in occur".....5k, 10k, longer?

    Thanks for the knowledge.
  • jim314jim314 Posts: 491
    Why would you want to spend a bunch of $$$ and risk screwing the vehicle up by mess'n' with it? Don't you think the vehicle and engine mfgrs' engineers knew a thing or two? Why not just use the vehicle as God meant it to be and spend the money and time doin' sumpn' that needs to be done?

    At least get a really good baseline on the performance as designed and built so that the true result of any mods will be determinable.
  • qbrozenqbrozen Posts: 22,075
    Honestly, I don't think those mods are useful on a diesel. What you need to get yourself is a reprogrammer. You can add sick amounts of power, then just set it back to stock for service visits. ;)

    '17 F150 Crew 2.7; '67 Coronet R/T; '14 Town&Country Limited; '09 LR2 HSE. 44-car history and counting!

  • From what I have been told, the technicians can see in the Cummins computer if it has ever been reset. And since a reprogrammer violates the Cummins warranty I am not going to mess with least not until I pass the 100k point.
  • I think the engineers at any given vehicle manufacturer, design trucks, engines, and parts to account for the lowest common denominator. For example the guy who changes oil when he remembers to, or the lead foot guy who loves to see if his truck can go 150 mph or cross that ditch......meaning that I think they over engineer things so that they don't break. I certainly believe that if you let an engine breath better that it performs better...otherwise you'd see catalytic converters and mufflers on race cars. This is my third Cummins and I love them. I am just interested in making it a little more efficient. But I won't do anything that could potentially hurt the engine like put a chip in it or change the turbo or anything like that. I want to switch to synthetic oil after the break in period, that's why I asked about the break in time frame. Don't get me wrong you make valid points and completely stock it is a great truck. Thanks for the info.
  • qbrozenqbrozen Posts: 22,075
    Have you ever changed the airbox and exhaust on your other diesels? In my experience, they are set up to breathe real easy from the get go. I mean, they HAVE to.

    Hmmm... i don't know about them being able to see if the ecu has been remapped. Seems unlikely, but I guess there is no point in taking a chance if it worries you.

    My father got a programmer for his GMC 2500 diesel. He's been running it steady for the past 80k miles on the middle program (can't remember what exactly the numbers are). He's never had issues with the warranty, and he even blew up the engine early on (not due to the programmer ... it was a known design defect that they repaired improperly ... long story). He tells me he gets better mileage with it ... real slight, though. I'm guess either placebo effect or just the engine breaking in.

    '17 F150 Crew 2.7; '67 Coronet R/T; '14 Town&Country Limited; '09 LR2 HSE. 44-car history and counting!

  • I changed the air box on my 2 F250 7.3 liters that I had but not any of the Cummins. Ford and CHevy are much more add-on freindly than Cummins is from what I understand. (Acouple of years ago I went to a local autoparts chain and tried to get a K&N filter for my 04 Dodge, I was told that K&N wouldn't even make an air filter for the Cummins for years due to some litigation from Cummins) Bottom line is that if the dealer that you buy your truck from is "power friendly" then you can get away with it, if not you are SOL. The CUmmins makes plenty of power for me right off the lot, my only concern is trying to increase the efficiency and perhaps get a little better fuel mileage.
  • qbrozenqbrozen Posts: 22,075
    I really don't believe you'll see mileage improvement with a CAI and catback. Cars are too heavily computer-controlled these days. Even if the add-ons move more air, the computer will add more fuel. It won't allow the mixture to change. So, IMHO, the only way to improve mileage with add-ons is by changing the programming.

    Otherwise, to improve your mileage, changing your driving habits is the only way.

    '17 F150 Crew 2.7; '67 Coronet R/T; '14 Town&Country Limited; '09 LR2 HSE. 44-car history and counting!

  • You make a valid point. And I always wondered how adding those two things supposedly improves power AND improves mileage. Seems contradictory.
  • ramownerramowner Posts: 1
    Is there any truth to a 6 speed stick getting better fuel economy than the Automatic?
  • ruking1ruking1 Posts: 18,382
    Yes for sure, all things being equal. In addition to a lesser loss of mpg, the stick also lets you better match the torque curve to the driving and road conditions. Since the torque comes on at much lower rpms, clutch wear can be lessen in that to engage 1st gear in most cases, you do not have to use higher rpms. The majority of times I fully engage first gear at idle.
  • likearocklikearock Posts: 13
    I purchased and installed the 60 gallon combination fuel tank/ tool box with the diesel install kit, which ties directly into your fuel fill nozzle to the factory tank and keeps the main tank full by gravity feeding for my 2006 2500HD duramax. I love the tank and the added range I get before re fueling. The only drawback I have experienced is My Chevy has the on board fuel computing and it gets confused since the main tank level never falls off full until the aux tank is empty, This has caused my fuel gauge to suddenly drop to empty when the tank is actually full, Once the truck is shut down for a little while the fuel gauge reads correctly again. I have started leaving my aux tank valved out and opening the valve when I stop and letting the main tank refill, then there are no problems with the fuel gauge getting confused.

    PS.. I have heard the dealer might be able to re program the computer to the new fuel capacity with the auxiliary tank but I have not personally checked this out.
    Hope this helps anyone who is considering installing an auxiliary fuel tank.
  • mongo65mongo65 Posts: 8
    I have thougth of doing that too. So many times you are out on the road and find a great deal on fuel and your sitting there wishing you had some spare cans or something. That programming sounds like a simple thing the dealer could do. If you don't mind me asking what brand did you get and how much did it cost you? I have a Dodge 2500 diesel with an 8 foot bed and I have thought of the drop in fuel box and just installing a larger tank underneath the truck so I don't lose any bed space.
  • likearocklikearock Posts: 13
    The 60 gallon auxiliary fuel tank/ tool box was right at
    $ 650.00 and the Diesel install kit was around $65.00.
    You can find regular auxiliary fuel tanks cheaper but I liked the look and it does not look like a fuel tank. I have the 6.5 foot bed and I have a slider 5th wheel hitch and don't have a problem, but you do give up some of you truck bed.
    Northern tools handles these RDS brand tanks, they are very well constructed and you can get a larger capacity if you like. WWW.RDS.COM
  • I have an 2006 Ram 2500 5.9 CTD, auto, 4x4, QC, SWB
    ONLY 5K miles on the odometer!

    I straight piped my exhaust, by basically removing cat and muffler and replacing with a straight piece of 4" pipe.

    YES, its loud... similar to a Harvester or a Tracter, BUT I am getting a solid 1.5-2 MPG better in mixed city driving with no load. Before my truck was avging 13-14 MPG now I am 14.8-15.4 MPG.

    Again, its loud... people will know when you leave and arrive whereever you go. I am in TX and straight-piped diesels are fairly common.. so in the 3 week since its been done I haven't been remotely hasseled by people or cops... that said YMMV, if you do it do it at your own risk.

    FYI, straight piping will void your exhaust noise and emissions warranty. So if I got a ticket for noise violation or altering the emissions of my vehicle.. Cummins and Dodge are NOT liable. That seems fair. Oh and keep your old stuff you never know when you might change your mind... Diesel cats are $300+, fyi.
  • jim314jim314 Posts: 491
    So you just don't care about the extra noise and hydrocarbon and NOx pollution from the tailpipe?

    How will you pass state inspection? Is straightpiping really that common in Texas? While riding my bicycle on some 2-lane country roads near Dallas I have had some young guys floor it to go past me in diesel pickups. There was a terrific cloud of dark smoky exhaust. Is this probably straight pipes or a legal stock exhaust system?
  • gordgord Posts: 5
    Good Post..
    My 2500 is same as yours but most of my milage has been in mountains/high passes (Colorado and AZ). On my last trip of 1800 miles from Mesa, AZ up through 8 and 10,000 ft passes in Colorado and return, pulling a 9000 lb FW, I averaged 13mpg. I now have 18,000 miles on my 04 and have been pretty happy with its pulling performance.

    Filling the tank with the foaming diesel has been a headache when I tried to get a good check on MPG. Usually takes about 10+ minutes to finally get the last 3-5 gallons in the tank. Have you had the same issue with the small filler tube in yours?

    Thanks for the post.
  • This sounds like a great set up. Where did you get the install kit?
  • I have a 2003 Dodge 2500 CTD, six speed and 4.10 rear end. About 85% of my miles are at 65 mph w/cruise pulling about 4000 lbs of trailer and livestock. New I would get 14.5 mpg. I took off the stock exhaust and replace it with a 4" MBRP turbo back exhaust and added a 4" Aeroturbine muffler. This pushed me up to 15.1 mpg. Then I purchased $7 of 4" drain pipe and cut a hole in the bottom of my air box to get more cold air by drawing it from down behind the front bumper and replaced the stock air filter with an Amsoil filter. This pushed me to 16.8 mpg. Next I switched to Amsoil motor oil and bypass filtration(to save $ on changes) which allowed my to get up to 17.1 mpg. Next on the docket is to change the diff, transfer case and tranny to Amsoil which I hope will allow more improvement. Other possibilities are an aFe torque booster tube to replace the stock air intake tube and possibly chipping the truck with an Edge chip set for fuel economy (this is a last step I someone can convince me of the fuel economy claims). Hope this helps. Each step in the above had between 10 and 20000 miles between doing so the mpg cover a pretty large window. The biggest improvement was the cheapest, being the modification to the stock air box.
  • Taxation is the main issue since the undyed has the road taxes. Farmers are given an exemptions with regard to sulfur because the government would bankrupt all farmers if they required farmers to go out and buy new $400,000 combines and $200,000 tractors. Because much farm machinery is 10 to 20 years old(sometimes older--my father in laws tractor was built in 1958) and these machines cannot use the ULSD without problems. And yes the prices are accurate.
  • Off road fuel has no highway taxes and they dye it pink so they can dip test your fuel tank but it does not produce a smoke I have seen it run in several trucks and it doesn't smoke at all.but be carefull if you are running it in a on road truck or even if you are driving equipment( bobcat,tractor) on the road the fine is severe.
  • rodougrodoug Posts: 5
    I'm new to diesels and am trying to determine what to replace my Chev 3/4 ton HD gas with. Would love to have car and truck but cannot afford it so this needs to be versatile. I drive to/from work ~40mi roundtrip plus haul a 8 - 11 thousand lb horsetrailer on weekends & vacations. I am wondering what the difference in mileage would be in mixed city/highway driving unloaded would be between the 4x2 and 4x4 and the 250 and 350. I am also wondering what it would be between straight highway speeds avg 70 loaded and unloaded. If anyone can share any info I would appreciate it. I can't find any official data anywhere and most of the Ford dealers have no personal experience. Thanks in advace.
  • KCRamKCRam Mt. Arlington NJPosts: 3,516
    Several more factors you need to consider...
    - cab size
    - bed size
    - terrain (flat, hills, mountains)
    - seasonal weather
    - tractional needs (winter, off-road, boat ramp)

    The best will obviously be the lightest - which would be an F250 regular cab 4x2; the worst is obviously the F350 4x4 crew cab dually. You'll need to determine what your cab needs are, what will be in the bed, and how and where you drive. Once you have a good idea what truck configuration is best suited, others can give you an idea of the mileage. Be sure to browse the Ford F-Series Real World MPG Numbers discussion as well.

    kcram - Pickups Host
  • rodougrodoug Posts: 5
    I have to have a crew cab due to 2-3 little ones in the back. I am pulling a gooseneck trailer and can use a long or short bed. Most of the time the bed will be empty. I'd love to have a duelly, but it's too big for all the in-town driving I do. The greatest percentage of mileage is a mix of city/highway mostly in the flat areas. I would rather have the 4x2 but I have heard the diesel is a heavy engine and you need a 4x4 if you will be in any soft dirt or mud. It doesn't snow much in Dallas so snowy weather isn't an issue. It does get pretty hot here in the summer, but I don't know if hot weather makes a difference. The Ford dealers I have spoken with have said that I could expect 2-4 mpg better from the 4x2 over the 4x4. Does that sound reasonable. I'll check out the other discussion as well. Thanks!
  • Hi, I have the 2008 f-250 crew cab diesel 4x4 short bed 373 gears lariat. I,m pushing 16000 miles on the truck. At 70 mph I get around 16.5 mpg solo. At 60 mph I get around 18mpg solo. I filled up today and got 14 mpg mixed . This is a 10 mile commute to work letting truck warm up 5 to 10 minutes in morning to defrost windows. city only I get 12.5 to 13 mpg. Pulling 9000 lb trailer with wife 3 teenagers, the bed loaded down with 5 bicycles lawn chairs fishing gear , grills camping stoves the works I got 9.0 to 9.3 mpg. this was at 65 to 70 mph and with only 3000 miles on truck. Pulling power is awesome set cruise on 70 and forget load is there. Highly recommend tail gate step and tow mirror option. Step saves a lot of wear and tear on back and folding mirrors makes driving thru bank atms and mcdonalds drive thru a whole lot easier
  • Thanks for the info. I really appreciate real world numbers.
  • Are you really having that good of fuel mileage with our 6.4L Diesel. I have the 08 XLT 6.4L Crew Cab, 20" aluminum wheels with almost 4000miles to date and I drive about 60% highway / 40% city and I just filled up today, 2-21-08, and my average was 11.8 mpg and thats at approx.70mph on the high tops. Now my 2005 6.0L Diesel was getting your kind of mileage but not my new 2008.

    I would be interested in conversing with you about your 08. my email address is

  • Hi John, If your truck does like mine your mileage should continue to improve for several thousand more miles. These engines are built with tight ring tolerances and take a while to break in. Also my driving is probably 75% rural 25% city and I drive consertive to get more mileage. I have also found that speed realy affects mileage. The other day on a long stretch of rural road I decided to experiment with mileage and speed. Using the mileage computer I dropped down to around 50 mph for about 5 to six miles and was showing around 24 mpg, 55 gave me 22 mpg, 60 gave me 20.5 and 65 was 19.3. Now I have never gotten as good of mileage as the computer shows when I calculate miles by gallons but I figured this would give me some ideal how much speed would effect mileage. Also I noticed that steep hills really effective mileage also. These are very heavy trucks. Mine weighs 7980 lbs empty with a full tank of fuel, so the amount of start and stops and hills will really affect individual mileage results. The price of diesel is hurting all of us but I,m still enjoying my truck. A couple of other things to watch is air pressure in tires I keep mine at 65 psig this is 18 inch rims and keep a check on the fuel water seperater. I do think you will see a 2 to 3 mpg improvement as you get more miles on your truck. Hope this info helps a little , will email you later. Thanks Frank
  • does your '08 warm up in the 5 to 10 minutes that you let it warm up. I am in the St. Louis area and my truck will not warm up when I first start it at home with my auto start. I have to be 10 miles down the road before it start blowing warm air. When it gets really cold I have to take it out on the highway to get it to warm up. Not to sure if this is normal for other 08 diesels or not. My '05 diesel that I traded in warmed up much quicker.

    Any info about your warm up time would be thankful
  • jim314jim314 Posts: 491
    I thought that not warming up at idle was a feature of almost all diesel engines. The diesel cycle allows such a lean burn that at minimum idle the engine simply does not make enough waste heat to keep the engine warm.

    I believe that some diesel engines can actually experience excess engine wear to the point of damage, if run at idle for an extended period--maybe an hour or two. The Mercedes inline-5 cyl diesel engine used in the Sprinter van actually had an option called "high idle", which I presume would be used to keep the interior warm if it had to be idled for an extended period say with a sighseeing application, or caught in a traffic snarl.

    So I don't think you should use a remote start to try to warm-up your diesel by idling it for 10 minutes. Start it up and drive away immediately. It would however, benefit from a block heater on a timer.
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