with that gearing going 2100rpms ur going too fast...ur mileage is about right IMHO on winter fuel and that rpm....not to mention us sound like ur timing is stock so ur not gonna start to loose optimal timing after 2000rpms...slow down to 1700-1800rpms on the highway and you'll get 18mpg

 

I would suggest the following: Complete fuel system cleaning (injector-out is best) and new fuel filter, also check and clean out or replace the overflow valve (search for this.) Replace the air filter as mentioned, and have a valve lash done.

It's still cheaper in the end to keep running the truck you now own, rather than loosing some serious $$$ in selling off a vehicle one month after you bought it.

 

I have a 98 12 valve 5 speed with 3.54 gear and I have been hand calculating the milage since I bought it with 31,000 miles on it. I have almost every fillup and the milage in a book in my truck. It only got around 16 mpg when I first got it and it gradually got better until around 60,000 miles. It will get over 20 mpg in the summer and it goes down to low 19's and high 18's in the winter.
I drive 27 miles each way to work and some of it is stop and go in the city. I usually keep it around 57 to 60 mph which keeps my rpms down around 1800 and I think this is where they get their best milage, maybe even around 1600 would be better but that is kinda slow. I have 154,000 on it now and I have never adjusted the valves or the timing. My son turned my starwheel some which gave it more power off the line.

 

I get better than 16mpg hauling 3000lbs of feed on half the trip and I definitely run higher than 2100rpm... more like 2600 with 3.54 gears in the hills of WV. Did it just last week to check the difference of a BHAF over the stock cleaner. BHAF is more than 1mpg better!!!! I am still not satisfied with my mileage, so I'll keep changin' stuff. And yes ask anyone who knows me and my truck, it gets really smooth and quiet above 84mph, soooo that's where we run.

 

Your mileage sounds low to meas well. My '96 will range from 14.5 towing about 12K to 19.5 not towing but with up to about a ton in the bed. The biggest thing I notice is to get this mileage I run around 1800 rpm on the highway. At that rpm I'm at about 68 mph.

 

I got 19mpg on my last tank, it was aggresive city/highway driving, meaning I boosted up every hill I could, and took off fast at every stop.

This next tank i'm gonna drive really conservative, accelling slow on takeoff, and idleing to stop lights. I wanna see what milage I get doing that for a couple hundred miles.

 

You should be getting much better mileage. I recommned do the following
1. Spend the $70 for a BHAF,part# NAPA 2790
2. Set the timing @ 15.5*, this the best timing for stock motor IMO. Make sure you have some who knows what they are doing when this is done. A Cummins shop shoulde able to do it pretty cheap.
3. Slide the cam plate all the way forward and turn the starwheel 2 full revolutions. If you search the ssite you will find this procedure. This is free power and will help the engine work less thus improve mpg.
4. Get someone to adjust intake and exhaust valves as stated above
5. Check the intercooler pipinig just to make sure the are no holes anywhere. This can causing a slight bogging of the engine down low. The rubber boot next to the fender on the right side in noturious for getting a hole from the fender.
6. Since you on stock power, plug the boost line to the turbo and get more boost. The HX35 has a max efficiency of 38psi. Running the boost up will also lower egts. Again this is a free mod.
7. Make sure the clutch fan is operating properly. A clutch fan that stays on constantly will draw power and mpg

Hope still will help.

Sarg
4.

 

Like the other people stated, with those gears at high speeds, your mpg will suffer. when I had those gears in my truck would get about 20 if I stayed below 70mph. Above that, it was drop very fast. Remember, the 4x4 has the aerodynamics of a brick! Since I tow and carry heavy loads, I change to 4:10's for my gears. Unloaded, I still get about 17mpg if I stay below 2100 rpm. If I want to go fast, I drive our Magnum.

Also, remeber that your GPS will indicate your linear speed, not your surface speed. It does not calculate for grade change (up and down hills). While a GPS may be accurate, elevation change will result in an error for surface speed calculation.

 

If the advice dealing with filters, valve adjustments, timing, etc. don't do much you might check out the wastegate actuator diaphram and lines leading to it. Your situation sounds a bit like what I went through with my '97 before the actuator diaphram failed completely, at which point mileage dropped to abysmal levels with almost no power anywhere in the rpm range. When everything is working right mine will pull 19 - 21 mph with 500 pounds in the back at a steady 70 - 75 mph.

 

Truck is in my sig, 650#'s of ballast in the bed and a 650# 8' plow on the front in strictly mtn driving varying 1600-2400RPM gets me consistent 19.7 with the blade and ballast gone its 20.2-21.5 I noticed my MPG go way down when my lift pump/fuel filter was heading south.

 

1700 is optimal. if you can cruise at 1700-1900 you'll get your best mileage over 2k rpm and the mileage drops significantly. also check timing. but 14 at 2100 rpm isnt bad i think i used to get 15 at that kind of rpm but at 1700 i would get 21-22 mpg. go figure

also is truck lifted
last time over flow valve was replaced? air filter. are they running winter blend fuel up there still?

 

The air filter was just replaced, along with the oil, just before I bought it (at a quick-lube place, unfortunately, so that means it has some scary kind of oil in it that's not Rotella.) I want to put a BHAF on it. It's not lifted, and we still have winter blend up here, but I don't use it--I always put in #2 and Power Service unless it's REALLY cold and then I might sneak in some #1. I try to avoid the blended stuff and/or the stuff with additives because I like to know exactly what I'm getting. I'm really tempted to start using a little 2-stroke oil because people seem to be having good luck with it. I use MMO in the First Gen because that's what the previous owner had always used.
Anyway, I slowed down to 65 and just filled it again and calculated it at 17.8. So things are looking up. I'm still going to take it in and have the timing checked, at least, and I just got a new fuel filter but haven't put it on yet.
Thanks everybody!

 

I just ran 300 miles on B99, mostly short trips and very little freeway driving and got 16.3 MPG in a 92 4x4, with a heavy steel bed on it. My bad tires are probably not helping me either.

 

Change your fuel filter, put in a fuel plate, and get a good air filter set up or a BHAF. My truck went from 14-15 when i bought it to 18-21 depending on if i was in Texas or Colorado and how hard I was pushing the truck. All I did was a fuel plate, gsk, BHAF, straight pipe. But definitely check the fuel filter.