I know our mileage is poor, but this is worse than I expected.
I drove 175 miles all highway, then about 10 around town then 175 home. I was going about 75 most of the time, not much traffic.
I have a magnaflow 3" cat back, no mechanical fan, synth oil, recent full tune up and 91 octane tune by Hemifever. I use 93 octane gas.

I figured I would get more like 17-18 mpg, but only 14! Is that poor enough to think I have some kind of issue, or is it normal? I need a 5th gear!

 

What rpm are you running at 75?

 

If I can recall, 2200-2300 rpm.

 

75 MPH with a 5000+ pound vehicle and a v-8 turning over 2 grand isnt exactly fuel efficient.... I bet it will go up at a slower speed....

 

I've got a stock 5.9l w/3.92 gears and average 15.3-16.2 @ 75mph depending on how many hills.

 

I used to get 15.8 on the freeway at 75 mph, but I was heavily modified 1998 5.9L with 3.92's and 31X10.5X15 tires.

 

I drove out to At Louis this summer, 85 at the slowest across I70, got about 13.5 MPG.

The best MPG I ever got in mine was 18.89 driving 70 from on SH85 to Colorado springs going about 65-75 the whole way depending on traffic. I drove around town down there for a while too, so I think on just the highway I had to be getting at least 20-21.

 

I tach 2k at 85! I have a 4.7 with the 45RFE with a 2002 jeep cherokee transmission control module. Just plug n play and you get a 5th gear. This only works on 2001 to early 2002 durangos, same year rams and dakotas with the 4.7 and 45RFE combo.

 

I'm in the same rpm range as you. Apparently the people i got my durango from did the TCM swap. It shifts thru 5 gears

 

I have the same setup, except mine is a 2002. I get 13-14 in town and 15-16 highway. I think my mileage suffers by about 1 MPG in the winter though. Not sure if it's the denser air or uglier driving conditions.