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| fuel economy... ** confused ** http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=5&t=76980 |
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| Author: | pwrwagn [ Mon Oct 07, 2013 9:50 pm ] |
| Post subject: | fuel economy... ** confused ** |
I just completed 1900 miles in 2 days. The slowest was the first segment, which was a combination of 55 and 65 mph roads. I got just over 23. The next segment was the fastest, with much of it having a limit of 80 and the rest 75, through the mountains. I got 25. The next segment was in Wyoming through Nebraska, and I got 25.4 - a speed limit of 75 mph. The last was across Iowa and Illinois, and I got 23.6, but it was the slowest and flattest. It really did seem that I get better mpg at 87 than I do at 67, which is absurd - since I drove from 5-7 mph over the limit the entire way. (how else do you get 1907 miles in two days?) |
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| Author: | tjkj2002 [ Mon Oct 07, 2013 10:13 pm ] |
| Post subject: | Re: fuel economy... ** confused ** |
Low engine rpm's does not mean better mpg's.The closer to the max torque output the better mpg's you will get.Your KJ was designed to get the best mpg's at highway speed which in most cases is 75mph. I always got my best mpg's at 78mph before I geared myself out of those speeds,I now get my best mpg's at 68 mph which my engine is spinning the same rpm's as before at 78mph. |
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| Author: | CRD Joe [ Mon Oct 07, 2013 11:09 pm ] |
| Post subject: | Re: fuel economy... ** confused ** |
tjkj2002 wrote: Low engine rpm's does not mean better mpg's.The closer to the max torque output the better mpg's you will get.Your KJ was designed to get the best mpg's at highway speed which in most cases is 75mph. I always got my best mpg's at 78mph before I geared myself out of those speeds,I now get my best mpg's at 68 mph which my engine is spinning the same rpm's as before at 78mph. Im not sure what planet ole KJ is from, but not only are you reving the crap out of the engine at those speeds but youre pushing the body (read brick) through the atmosphere creating WAY MORE drag the engine has to fight. Ive gotten my best MPG at 57mph. Not only is the engine purring at low rpm (read less fuel being sent to the engine) but the engine isnt working as hard trying to push the truck through the atmosphere. Ive gotten as high as 39.80 mpg driving like that, but 36-38 is normal. Diesel engines are VERY RPM sensitive. You get your best MPG at lower RPMs. This philosophy has worked very well for me. 2001 VW TDI 62MPG best 58 is normal. 2005 CRD KJ 39.80 MPG best 36 normal 1998 Dodge 12V cummins 28MPG best 25 normal. |
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| Author: | tjkj2002 [ Mon Oct 07, 2013 11:20 pm ] |
| Post subject: | Re: fuel economy... ** confused ** |
CRD Joe wrote: tjkj2002 wrote: Low engine rpm's does not mean better mpg's.The closer to the max torque output the better mpg's you will get.Your KJ was designed to get the best mpg's at highway speed which in most cases is 75mph. I always got my best mpg's at 78mph before I geared myself out of those speeds,I now get my best mpg's at 68 mph which my engine is spinning the same rpm's as before at 78mph. Im not sure what planet ole KJ is from, but not only are you reving the crap out of the engine at those speeds but youre pushing the body (read brick) through the atmosphere creating WAY MORE drag the engine has to fight. Ive gotten my best MPG at 57mph. Not only is the engine purring at low rpm (read less fuel being sent to the engine) but the engine isnt working as hard trying to push the truck through the atmosphere. Ive gotten as high as 39.80 mpg driving like that, but 36-38 is normal. Diesel engines are VERY RPM sensitive. You get your best MPG at lower RPMs. This philosophy has worked very well for me. 2001 VW TDI 62MPG best 58 is normal. 2005 CRD KJ 39.80 MPG best 36 normal 1998 Dodge 12V cummins 28MPG best 25 normal. Guess you actually need to understand how a engine works,more so a modern engine. Your older non-computer controlled diesels in the medium and HD trucks yes low rpm's are best but they only rev'd to 2800rpm's in the 1st place and make there max torque at a much lower rpm.Your 2.8 CRD rev's well over 4000rpm's and your max torque rating occurs at a much higher rpm then the big diesels. Then again you really don't own or drive a diesel for what a diesel does best,towing.I can tell you your 12v never got 28mpg's,not possible unless you turned off the engine and coast in "N",which with your driving style I wouldn't put it past you. |
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| Author: | pwrwagn [ Mon Oct 07, 2013 11:29 pm ] |
| Post subject: | Re: fuel economy... ** confused ** |
tjkj2002 wrote: Low engine rpm's does not mean better mpg's.The closer to the max torque output the better mpg's you will get.Your KJ was designed to get the best mpg's at highway speed which in most cases is 75mph. I always got my best mpg's at 78mph before I geared myself out of those speeds,I now get my best mpg's at 68 mph which my engine is spinning the same rpm's as before at 78mph. If you study the effects of aerodynamics and the force required to move an object through air, the power required is not a simple straight line. It is a dramatic upward swing as the speed climbs, which means the power required to move the jeep at 85 is vastly more than it takes to move 70, which is again, vastly more than needed to go 60. The equation for drag SQUARES the speed. That is 10 squared is 100 but 12 is 144 and 40 is 1600. This change from 65 to 85 is a doubling of air drag and at anything above 50 mph or so, aerodynamic drag is the largest horsepower consumption, on flat ground. If you get more MPG at 85 than 65, or even if it is the same, then the inefficiency at 60 is just plain absurd, meaning that the change in efficiency - expressed as BSFC (brake specific fuel consumption) - must be massive. That would be extremely unusual for a diesel motor, which does not suffer the pumping losses that make gasoline engines inefficient at low speeds and light loads. |
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| Author: | pwrwagn [ Mon Oct 07, 2013 11:33 pm ] |
| Post subject: | Re: fuel economy... ** confused ** |
tjkj2002 wrote: Low engine rpm's does not mean better mpg's.The closer to the max torque output the better mpg's you will get.Your KJ was designed to get the best mpg's at highway speed which in most cases is 75mph. I always got my best mpg's at 78mph before I geared myself out of those speeds,I now get my best mpg's at 68 mph which my engine is spinning the same rpm's as before at 78mph. Actually, "peak torque" is not the definitive maximum economy. An engine's BSFC (brake specific fuel consumption) can be rather flat, or it can have a dramatic peak or even multiple peaks over it's rpm. Peak Torque is simply the point where the induction system, thermal efficiency, and fuel delivery reach their combined optimum. And it may be far from the maximum fuel efficiency. Perhaps this is what GDE stumbled on to improve fuel economy. |
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| Author: | thermorex [ Mon Oct 07, 2013 11:41 pm ] |
| Post subject: | Re: fuel economy... ** confused ** |
My best mileage was little over 25 recently. I get same mileage city drive and highway. I drive very low on throttle in city and normal on Highway (usually 8-9 over speed limit on cruise). The mileage on Highway is not that good due to the non aerodynamic shape of the car, at least that's my conclusion based of getting same mileage both city and highway. I used to get 22 city with27-30 highway prior to having offroad tires (not oversized), Grille guard and mopar light bar. To the point, could be more wind blowing from front and/or slight unnoticeable uphill. |
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| Author: | joelukex4 [ Mon Oct 07, 2013 11:49 pm ] |
| Post subject: | Re: fuel economy... ** confused ** |
CRD Joe - I would love to be able to get that mileage but I just can't drive that slow. I will say that I was getting 29 - 30mpg this summer before the GDE tune. Now that I got the Eco tune I only get 28 mpg at 75 mph. I want to know why. Everyone said their mpg went up after the tune but mine went down. It does run better though. Before everyone says I put my foot into it more, this is cruise control on the interstate for 400 miles. |
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| Author: | tjkj2002 [ Tue Oct 08, 2013 12:10 am ] |
| Post subject: | Re: fuel economy... ** confused ** |
pwrwagn wrote: tjkj2002 wrote: Low engine rpm's does not mean better mpg's.The closer to the max torque output the better mpg's you will get.Your KJ was designed to get the best mpg's at highway speed which in most cases is 75mph. I always got my best mpg's at 78mph before I geared myself out of those speeds,I now get my best mpg's at 68 mph which my engine is spinning the same rpm's as before at 78mph. Actually, "peak torque" is not the definitive maximum economy. An engine's BSFC (brake specific fuel consumption) can be rather flat, or it can have a dramatic peak or even multiple peaks over it's rpm. Peak Torque is simply the point where the induction system, thermal efficiency, and fuel delivery reach their combined optimum. And it may be far from the maximum fuel efficiency. Perhaps this is what GDE stumbled on to improve fuel economy. On paper,in the real world not so much.Just spent a whole week on the topic last week. Now this don't apply to diesels but in a modern FI engine simply taping off about 75% of the air filter will result in increased mpg's,lower power but the mpg's will go up about 1-2mpg's.Now on a diesel since nothing is really monitoring the exhaust it can not do that. |
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| Author: | pwrwagn [ Tue Oct 08, 2013 12:18 am ] |
| Post subject: | Re: fuel economy... ** confused ** |
tjkj2002 wrote: Now this don't apply to diesels but in a modern FI engine simply taping off about 75% of the air filter will result in increased mpg's,lower power but the mpg's will go up about 1-2mpg's.Now on a diesel since nothing is really monitoring the exhaust it can not do that. Makes sense. Since the valve overlap is very high, and opening the throttle at higher rpm's can and will result in incoming air/fuel mixture actually filling the cylinder and a little going out the exhaust valve, cutting down the intake airflow will result in less incoming air, meaning less can go out the exhaust valve. |
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| Author: | mass-hole [ Tue Oct 08, 2013 6:20 pm ] |
| Post subject: | Re: fuel economy... ** confused ** |
tjkj2002 wrote: Low engine rpm's does not mean better mpg's.The closer to the max torque output the better mpg's you will get.Your KJ was designed to get the best mpg's at highway speed which in most cases is 75mph. I always got my best mpg's at 78mph before I geared myself out of those speeds,I now get my best mpg's at 68 mph which my engine is spinning the same rpm's as before at 78mph. I have been getting this intense feeling that I get better MPG's as i push the speed up. I seem to do better between 70-80mph than I do at 65mph and no one believes me when i tell them. |
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| Author: | thermorex [ Tue Oct 08, 2013 6:31 pm ] |
| Post subject: | Re: fuel economy... ** confused ** |
I think the throttle and air drag are the worse enemies for good mileage. But definitely going uphill a steep way with low rpm may cause worse mileage than going closer to max torque, and I believe few hard-core tow ppl even mentioned increasing the rpm not only for better torque, but also for better mileage |
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| Author: | tjkj2002 [ Tue Oct 08, 2013 6:52 pm ] |
| Post subject: | Re: fuel economy... ** confused ** |
thermorex wrote: I think the throttle and air drag are the worse enemies for good mileage. But definitely going uphill a steep way with low rpm may cause worse mileage than going closer to max torque, and I believe few hard-core tow ppl even mentioned increasing the rpm not only for better torque, but also for better mileage A little more rpm's helps in cooling also. Now diesels do not have throttles,you may have a butterfly in that TB but's it's not to control how much air is entering the cylinders.It's there to actually maintain a slight vacuum in the intake so the EGR functions properly and charges your vacuum res for your vacuum assisted brakes. |
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| Author: | mark2m [ Tue Oct 08, 2013 10:12 pm ] |
| Post subject: | Re: fuel economy... ** confused ** |
mass-hole wrote: tjkj2002 wrote: Low engine rpm's does not mean better mpg's.The closer to the max torque output the better mpg's you will get.Your KJ was designed to get the best mpg's at highway speed which in most cases is 75mph. I always got my best mpg's at 78mph before I geared myself out of those speeds,I now get my best mpg's at 68 mph which my engine is spinning the same rpm's as before at 78mph. I have been getting this intense feeling that I get better MPG's as i push the speed up. I seem to do better between 70-80mph than I do at 65mph and no one believes me when i tell them. Your not the only one. I drive between 70-80 mph usually at 75mph and according to my EVIC I average around 30mpg and have gotten 37mpg. Yes I know it's not accurate. But it is better at those speeds than at 50 - 60 mph. I did hand calculate a cross country drive towing a trailer maintaining 75 and average 18mpg. I'm happy with that. No tune at the time bone stock. |
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| Author: | dskater411 [ Wed Oct 09, 2013 7:14 pm ] |
| Post subject: | Re: fuel economy... ** confused ** |
I drive 60 miles each way to work, 90% highway, and always try to maximize my fuel economy. My experience is keeping it around 65mph yields 30mpg and 75mph yields 25-27mpg. Also how much I am depressing the gas peddle and how often has a much greater affect than rpms for me. As long as I press the peddle as light as possible i.e. losing speed up a hill a bit and picking up speed then coasting down a hill, things are good. Another interesting side note is a long road trip up a mountain and back down again (e.g. going snowboarding at mammoth from san diego) gives me better economy than long flat highway cruising. Several years ago I had an e46m3 and at 65mph only gave me 25mpg but 85mph I got 28mpg, so I definitely think there is something to cars running more efficient at different engine rpms but I think the CRD's boxy characteristics outweigh that in this case, at least for me personally |
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| Author: | usa591 [ Thu Oct 10, 2013 3:35 am ] |
| Post subject: | Re: fuel economy... ** confused ** |
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| Author: | SargeIndustries [ Thu Oct 10, 2013 3:17 pm ] |
| Post subject: | Re: fuel economy... ** confused ** |
for those of you who have a Scan Gauge and a exhaust temperature gauge, I think you will notice as the RPMs are around 2000, the EGTs read lower and the gallons per hour(GPH) also read lower than if you lug your CRD. Low RPMs = lower mileage unless there is something unique about my CRD.... |
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| Author: | dirtmover [ Thu Oct 10, 2013 3:27 pm ] |
| Post subject: | Re: fuel economy... ** confused ** |
SargeIndustries wrote: Low RPMs = lower mileage unless there is something unique about my CRD.... So for best mpg we should all hit the OD off button and redline it on the highway? 
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| Author: | usa591 [ Thu Oct 10, 2013 3:36 pm ] |
| Post subject: | Re: fuel economy... ** confused ** |
FWIW, when I put A/T Geolanders on to replace the stock Goodyears, I noticed a -2mpg difference on average. I'm a little underwhelmed with my fuel economy, however I will admit that I think I have a thermostat that is in "frozen open position" as my engine temp. gauge usually sits on the low side between the 1/4 and 1/2 marks. Here in California, I'm usually driving around in 72' weather. |
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| Author: | papaindigo [ Thu Oct 10, 2013 4:03 pm ] |
| Post subject: | Re: fuel economy... ** confused ** |
usa591 no need to "think" about it; your tstat is opening early and costing you ca. 10% in mpg. Been there done that. |
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