I agree that 'in most cases' 4.56 gears will cause the highway cruise MPG to go down.
Almost certainly if a 3.21 or 3.55 diff gear Ram follows a equal weight and equal tired 4.56 diff Ram on the same highway at the same speed under the same wind conditions, the 'taller' gear Ram will get the better MPG, in the range of 0.5 to 1.5 better.

In the city, the 3.21 or 3.55 Ram will get slightly better MPG than the low geared Ram - unless driver behavior is different - and it almost always is.

City driving is dominated by:
1. How fast you accelerate - you typically get 3-6 MPG while accelerating.
2. How much you idle - you get 0 MPG when idling.
3. How much you brake - braking is actually 'negative' MPG.

This is related to the often debated idea of whether OD should be turned on or off in city driving.
below is an old post where I tried to experiment on that subject:
---
Here's the results of several tanks of gasoline I kept track of with the OD either on or off in city driving:

I have since made 3 more 300 mile long mpg tests in city driving since i posted the original 6 tests. My test results now look like this:
-----------
OD turned off runs:

7-19-00 14.32 mpg Amoco87
7-29-00 15.17 mpg Amoco87
8-08-00 14.23 mpg Amoco87
9-08-00 13.53 mpg Amoco87
---------------------------------
average 14.31 mpg

OD turned on runs:

7-12-00 14.16 mpg Exxon87
8-17-00 14.25 mpg Amoco87
8-24-00 15.49 mpg Amoco87
9-01-00 15.03 mpg Amoco87
9-12-00 14.35 mpg Amoco87
-------------------------------
average 14.66 mpg

As you can see, so far the OD on/off difference seems to be small with a slight 0.3 MPG edge to leaving OD on while in city driving. That is about a 2% difference and probably not enough to be 'statistically significant'.

Note also how much the MPG jumps around. All realworld tests have 'random variation.'

If I had done just two tests, and had by chance gotten the low 14.16 for the OD on, and the high 15.17 for OD off, I would have falsely concluded that OD on really hurts in city driving.

To really find reliable MPG numbers for Rams in city driving you need to do multiple tests.

This is on a 5300 lb Ram ClubCab 2wd with the 46RH auto, the 5.9V8 and a 3.21 diff ratio.

 

from osteodoc's request earlier:

Back in model year 2000
Ram 1500 4x4 had a
Cd of 0.48 and
frontal area of 35.34 square feet

Compared to KC's post today:
2006 1500 Quad 4x4 short box:
frontal area: 34.9 sq ft
drag coefficient: a bricklike .0528

Back in 2003
KC Jackson provides info that:

1500 Quad 4x4 (only Cd listed)
Cd = 0.528 -
frontal area
4x2 SRW - 34.6
4x4 SRW - 34.9

2500/3500:
drag coefficient
- none listed
frontal area
4x2 SRW - 35.1
4x2 DRW - 36.8
4x4 SRW - 35.4
4x4 DRW - 37.3

>The Cd increase makes sense based on the larger frontal area. The new
>Ram is taller (up to 2 inches) and slightly wider (about half an inch)
>than the 1994 design.
---------

in 2000 model year
the most aero Ram was the
ClubCab 4x2 1500
Cd = 0.44
Frontal area = 34.49

 

I have the cure for poor mileage,A couple of days ago I was driving down the road in my Hemi ,waiting for the prices to go up again when I came upon the answer. Along the side of the road was this cute red Oldsmobile coupe just sitting there with a for sale sign on it. It's a quad 4 Two door coupe. The car only cost $2400.00 and it was a 92. Not a scratch on it or even a dent, and the best part it only had 36,000 miles on it. So I did the only smart thing since getting the Hemi, I bought The Olds.The old lady that sold it to me had the original receipt and all the receipt's for service. So it really does only have 36,000 on it. The sticker says 24 city 36 Hyway. So if the price of Gas goes up to $4.00 or above My wife gets the Olds I get the Buick Centry and The Hemi get's parked. The Buick gets 23 city 34 Hyway. The Hemi is full of gas and I only need it to pull my Boat anyway. so We'll see what happens next week.

 

Thank you HankL. That would explain why I can only get my truck to do 108+or-. It get's there fast but just can't seem to gain more speed after that.