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Fuel mileage display question.
http://www.lostjeeps.com/forum/phpBB3/viewtopic.php?f=5&t=4751		 Page 1 of 1
Author:  BodhiBenz1987 [ Sat Jan 07, 2006 8:39 pm ]
Post subject:  Fuel mileage display question.
How is the fuel economy calculated for the numbers displayed in the menu panel? I'm just curious because mine tends to be a little erratic and I'm having a hard time believing some of the numbers it gives me. It's telling me 16.5-17.5 right now in somewhat stop and go traffic, but nothing severe. I know these vehicles have been known for disappointing mileage, but the reason I question it is that when I manually calculate mileage (the old fashioned way) I get much better numbers (21 was the worst; 27 the best). I only discovered the digital monitor about a month ago ... before that I was doing it manually. Last tank I tracked with the display, it told me 17.2 for the first half, which I did in around-town driving. I used the second half on a 200-mile highway trip ... and at the end of the tank it said 25 mpg. Mathematically that suggests awfully GOOD mileage for that second half. What gives with this thing? How does it calculate economy and is it fallable?
Author:  dog_party [ Mon Jan 09, 2006 12:30 pm ]
Post subject: 
The only thing that will give you accurate results is you calculating it youself (miles driven - fuel used). Don't put more than passing interest in the computerized display of MPG. The system must begin it's fuel-mileage calculation based on it's understanding of the amount of fuel in your tank, and we know that F is not as full as it gets and E does not technically mean empty. My EVIC says "0 miles to empty" when the needle hits E...I can still drive 30+ miles after it passes E so any calculation that it tells me is invariabley going to be wrong.

Just my 2 pennies.
Author:  LibertyCRD [ Mon Jan 09, 2006 2:53 pm ]
Post subject: 
the overhead computer in my '04 dodge truck was PERFECTLY accurate. i reset it each time i filled up the tank, and it always read exactly perfect when I checked it by calculating manually. so i learned to rely on the overhead computer, as long as you reset it with each fillup.
Author:  Ranger1 [ Mon Jan 09, 2006 7:15 pm ]
Post subject: 
My 05 CRD EVIC consistently reads 2mpg less than it actually gets, ocassionaly it reads 3 mpg less. My 02 Liberty V6 EVIC reads accurately to within .2 mpg most of the time... I don't think the CRD EVIC is calibrated very accurately.
Author:  BodhiBenz1987 [ Mon Jan 09, 2006 8:50 pm ]
Post subject: 
Is it calculated based on the amount of fuel in the tank or the amount of fuel passing through injectors? I can think of different ways the economy could be calculated, but was wondering how it actually works. Doesn't day anything to that tune in the owner's manual, and I don't have any tech literature beyond that.
Author:  Ranger1 [ Mon Jan 09, 2006 9:03 pm ]
Post subject: 
I don't know!!! I can guess, but have no data on how they do it. One would guess that the designers know the amount of fuel injected based on rpm and load, and just calculate the fuel usage against the odometer and come up with a mpg figure. However, I don't think that would take into account the fuel being returned to the tank. At least on mine, their calculations don't work as well as my 02 did. I wonder if they used the same program for the V6 gas engine - which wouldn't quite be the same as diesel due to fuel density, fuel btu per gallon, engine efficiency and so on. All I know for sure is that my EVIC has been reading low since day 1.
Author:  awindj [ Wed Jan 11, 2006 10:55 pm ]
Post subject:  Display vs calculated mileage
Over the last 9 tank fills, I have recorded the display mileage and the calculated mileage. The display has averaged 23.7 MPG and the calculated mileage has been 22.7.

The last tank the display read too high by 4.8 MPG. My best calculated MPG was 26.5. I have not been overly impressed with the mileage. I do have a lift kit that I am sure has some impact. I now have about 13,000 miles.
Author:  oldnavy [ Thu Jan 12, 2006 12:35 am ]
Post subject: 
If you have larger tires with the lift it will slow the speedometer down and show less miles traveled then actually traveled. With that your MPG's also go down because you are showing less miles traveled per gallon of gas used then you are actually traveling.

The EVIC work's by knowing how much fuel is burned at idle (it programmed in) and by throttle imput (it drive by wire) through the CPU, sometimes it can be off because the IP (Injection Pump) is a little out time and that screws up everything.
Author:  Taz [ Thu Jan 12, 2006 7:57 am ]
Post subject: 
My EVIC is right on if I hand calculate it for a whole tank of fuel.
Author:  BodhiBenz1987 [ Thu Jan 12, 2006 5:21 pm ]
Post subject: 
Taz wrote:
My EVIC is right on if I hand calculate it for a whole tank of fuel.


Last time I filled up, it read just over 24 on the EVIC ... so maybe it is correct when I've used a full tank. But I'm guessing the estimates given on the EVIC when the tank it half full aren't too great. It told me at the half-tank point that I was getting 16.7 mpg. By just under 1/4 tank, it had reached over 26, which implies I was getting significantly better than 30 mpg on parts of that second tank, mathematically. Hey, I'm a good driver in terms of fuel economy, but not that good ...

I think I'll just trust my little calculations, like I've bene doing for years on the Benz. 8)
Author:  Flying J [ Sun Jan 15, 2006 12:48 am ]
Post subject: 
The computer on the CRD always is about 2-3 MPG lower then hand calc'd mileage. I got spolied because the computer on my Ram Cummins Diesel is always within less then .5 MPG different.
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