Engine/Transmission (1998.5 - 2002) MPG not what they should be

[onestackdram]

My wifes '02 just does not seem to be getting the MPG's that most of you 24v ers are getting. Her avg is only 14. 5 that either on the interstate and in the city. And she is extremely easy on the trottle. I know that most of you guys are getting anywhere from 18 to 20 give or take. So I am thinking that something must not be right. I know on my 12v when my MPG went down hill it was simply a matter of working on my overflow valve. But I am new to these overvalved things. So my question is, is there something similar to check? I just changed all the filters on it so I know that those arent the problem.



TIA



Russell

 

[Scrappy]

mpg

i'd check the map senser, if it is dirty, your mpg will go down.

 

[OTRPU]

and all other usual things...

Like change or clean the air filter, change the fuel filter, these simple items can account for poorer mileage.



Be Safe,

Steve J.

 

[ECappleman]

Are you basing milage off the trip computer or gas tank refill? My trip computer mpg calculator is 2-3 mpg low.

 

[onestackdram]

her overhead console comp seems to be in line with what she has been getting. we have tested both ways.



russell

 

[andrewkoperdak]

I did not see your ring/pinion gear ratio in your signature. Many many trucks running 4. 11:1 ratio get the same MPG that you report. Your truck weight may be higher than some other also. I, for example, have a very light truck at 6200 pounds. I get 26 MPG highway and 22 city (calculated). My trip computer is off by 2. 5 mpg so I don't use it for my numbers, it just to tells me if something is going wrong. Anyway, @ 70 I am only making 2000 rpm. I am betting yours is higher and your vehicle weight is higher.

R,

Andy

 

[Sticks]

Originally posted by onestackdram

And she is extremely easy on the throttle.

There is your problem. She needs to DRIVE the truck, not nurse it along. I have been getting my best mileage with firm acceleration, and shifting at 2400rpm, and trying to keep the cruising rpms at around 2100.



You may as well clean the AIT sensor while you are under there.

 

[onestackdram]

well the '02 has the 3:54 R & P and when I say light on the throttle you have to look at what I drive and add the new toys that I havent listed on it yet so my idea of easy may be warped. But you guys are gonna have to inform me as to where those sensors are cause I might as well be reading japanesse when i look under her hood.



Russell

 

[Rogue Ram]

First off, what miles are on the truck, and has it always gotten this mpg?



My 99 auto 4x4 with a 4:10 without the VA box will easily get 18-19 no sweat. If the truck isn't broken in relatively well, it can make a difference. Check all of the above, then make sure you are filling up with regular #2 diesel, and not some leftover tank of winterized stuff. Get the fuel pressure checked, I've yet to see too much of anything knock mpg down that far that wasn't lift pump related, nevertheless, check all the relative filters, and check the links on this for the link to Fritz's site to see where the MAP and IAT sensors are. There should be plenty of info on that site and this one as to what to do regarding these sensors.



That mpg isn't right, but you'll never get a dealer to do anything about it.



R

 

[RonaldLarson]

More checks

First, have you changed to larger diameter tires? I went up one size and it equates to 6. 7 % error in favor of fuel mileage. Second, the valve lash could be out of adjustment. This makes a pretty good difference in mileage, too.

I get 21+ at 75 highway, and 17+ city. '99 quad cab auto, 3. 55, camper top, weight 7660 full of fuel, two aboard.

Ron

 

[WadePatton]

Normal lead-footing around gets me 18-19mpg and EASY on the throttle got me 22. 5 at around 7000#.



And the mileage should increase some up to 100k or so.



The IAT and MAP sensors on the left side toward the back.

 

[onestackdram]

Ok I figured out where the sensors are at and when she gets home. As far as the tire size change it has 285's BFG's on it and since we bought the truck used I am not sure what the OEM size is. But I hadnt' thought of that, but that makes sense. So if that is size is way different what is the formula to figure out what the discrepency is? I ask this because the speedometer is right on target with where it is supposed to be. I have checked it several different times using different methods.



Russell

 

[WadePatton]

Okay if the tires are 285 and the speedo NOT recalibrated that would explain it. But you say the speedo is right. The factory computer will go that big I'm told.



I use 10-mile stretches of marked highway to check speedos. Some folks use GPS. I haven't tried that yet.

 

[RonaldLarson]

OK, now we can fix it

You need to subtract the new tire diameter from the old tire diameter. Now divide the sum by the new tire diameter. This will give you the percentage difference between the old and new tire. Now simply multiply your odo reading times the percentage, and add the percentage to the odo. Shazam, the correct odo mileage. I verified mine with GPS, and my figures were right on: my speedo/odo were 4. 9 miles short. Example.

200 miles X 6. 7%= 13. 4 miles. 200+13. 4= 213. 4 miles.

There's your true MPG.

Ron

 

[bmoeller]

mph vs. rpm calc. - http://www.f-body.org/gears/

 

[geusterman]

I just ran a long string on this subject of lousy mileage for 2002 4x4's with no lockout front ends. See: https://www.turbodieselregister.com/forums/showthread.php?threadid=77095

and another one re the front ends. I wonder if it is the ECM. Or the VP44. One fellow got a valve adjustment that helped some. Some 2002 4x4's do allright so it was not the front end. I will pursue this as time allows. geusterman

 

[geusterman]

Here is the other string on the non-disconnect front ends and their mileage issues. https://www.turbodieselregister.com/forums/showthread.php?threadid=77451

geusterman

 

[Mtngoat]

Russell-



How'd this turn out? Just FYI, my truck got 18. 2 coming back from Denver this week at 70 mph with the A/C on. You should be better than that. I am still jealous of the guys getting 20+!!!



Robin

 

[WadePatton]

19. 3 last tank with the box on 4x5 or 5x5 the ENTIRE 600 miles. Rural highway, no interstate driving. It just keeps getting better.



DD3's go in tomorrow. I expect mileage to plunge for a few weeks.