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Pickups Diesels Diesel Fuel Economy and chips
391 messages, Last post on Oct 28, 2009 at 4:28 AM
You are in the Diesels Forum. Your Host is kcram
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Replying to: frankoduramax (Apr 10, 2006 8:59 pm) |
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I have a 2006, quad cab 4x4 3500 Cummins diesel, with 315 off-road tire. I have been getting poor fuel economy from day one, 12.5-14.5. Most of my driving is highway driving in the Texas hillcountry. I started using a superchip flashpag re-programmer after about 6,000 miles. Once I started using the re-programmer I started checking the actual mileage, from gallons to fill up/miles on odometer, vs the mileage the onboard overhead computer is telling me i am getting. I found that the mileage the computer was telling me was vastly different from what i actually measured. Using the superchip "Performance" program the computer would tell me I was getting 22-24mpg on the highway and 20-21 average around the ranch, highway. I was actually getting 15-16 average. The superchip programmer lets you adjust for the larger tire size so assuming this works the speedometer and odometer should be adjusted for the larger tire size. I recently went on a long highway trip from central Texas to Colorado Springs. I checked the mileage at every stop and was consciously keeping my speed under 70 in an effort to maximize my fuel economy. I had the computer set to the Superchip performance program. Same results, the onboard computer said I was getting 22 mpg but the measured mpg was never better than 16, usually 15 or so. I have recently set the truck back to the stock program. The computer tells me I am getting 15 mpg or so but I am actually getting 12.5 mph or so. I don't have a lot of confidence that the dealer will be motivated to find the cause of my poor fuel economy. Any help, insight or suggestions will be appreciated. dwd
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Replying to: dwd (Aug 20, 2006 10:20 am) Have you checked the accuracy of your odometer? You can use marked off highway segments or use Google or Yahoo map distances and compare. Does your speedometer agree with the unmanned radar installations, "Your vehicle speed is ____."? I have heard people disparage the accuracy of vehicle computers. The only vehicle I have checked is a 2001 Jeep Grand Cherokee Ltd. with 4L I-6 gas engine belonging to a relative. On a trip I found that my calculation from odo and fuel added at the pump agreed extremely closely with the computer average.
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Replying to: jim314 (Aug 20, 2006 11:15 am) dwd, you're probably not going to get much better mileage than you have achieved. My 05 Cummins is consistently 10-15% worse than my 96 Cummins was. Plus, those 315s have raised your unsprung weight and rolling resistance substantially and you're using extra horses to get that rubber moving. With my 05 (3500 Quad 4x4 dually), the only way I can clear 20 mpg is at a steady 52 mph. kcram - Pickups Host
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Replying to: jim314 (Aug 20, 2006 11:15 am) |
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Replying to: kcram (Aug 20, 2006 4:13 pm) dwd
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Replying to: kcram (Apr 24, 2006 6:15 pm) They both have about 12000 miles on them and are in great shape, the 04 is $29800, the 05 is $31700. I like the color of the 05 ( more warranty left on it)but I have heard that the early 04s have the older engine and gets better fuel mileage. I've heard the 05s only get about 14-16 empty, and 12-14 loaded(depending on what you are towing of course). I've also heard the 02-early 04 engine will get 20+ empty and 16mpg towing. Is this true??? Can you add the locking lugs and Quadzilla chip to the 05 and get the fuel mileage compareable to the early 04?? Thanks for any help, I've never owned a diesel and need some help. Any issues I should be aware of with either engine??
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Replying to: snowmanx (Aug 20, 2006 10:03 pm)
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I've narrowed it down to a early 04 2500 4x4 QC short box or a 05 2500 4x4 QC short box, both are pretty much identicle with options, both are SLT, tow, single cd, power every thing etc. They both have about 12000 miles on them and are in great shape, the 04 is $29800, the 05 is $31700. I like the color of the 05 ( more warranty left on it)but I have heard that the early 04s have the older engine and gets better fuel mileage. I've heard the 05s only get about 14-16 empty, and 12-14 loaded(depending on what you are towing of course). I've also heard the 02-early 04 engine will get 20+ empty and 16mpg towing. Is this true??? Can you add the locking lugs and Quadzilla chip to the 05 and get the fuel mileage compareable to the early 04?? Thanks for any help, I've never owned a diesel and need some help. Any issues I should be aware of with either engine?? |
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Replying to: dwd (Aug 20, 2006 4:30 pm) Therefore your true mpg values would be 1.087 times what you have reported. So, e.g. 19.0 mpg from an uncorrected odo reading would actually be 20.7 mpg. And if your speedomoter read 70 mph you were actually going 76 mph. The power required to overcome air resistance rises strongly with increasing speed (NASA) so the engine was having to put out more power at the wheels which would act to decrease true mpg. It is true that these larger tires will decrease engine rpms by 8.7% at the same true speed and this could improve fuel economy especially on level ground, lightly loaded, but it almost certainly would not balance out the increased air resistance, and it would definitely hurt hill climbing performance and pulling force in any gear. Also these wide off-road tires are going to have large rolling resistance compared to tires designed for pavement. If you are really interested in getting maximum mpg then think about the tires you replace these with, but don't go overboard on getting low rolling resistance tires that don't have good traction, especially on wet roads. It does rain in the Texas Hill Country occassionally, doesn't it? You don't want to slide off the road into a gulley some rainy night when a deer jumps out in front of you. Wouldn't be good for your truck, for you, and maybe wouldn't even save the deer. Calculations: Dia 315/70-17 = (315mm)(1in/25.4mm)(0.7)(2) + 17in = 34.36in Dia 265/70-17 = (265)(1/25.4)((0.7)(2) +17 = 31.61 in 34.36/31.61 = 1.087.
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