depends on alot of different factors the biggest one being the wind strength and direction i would imagine against the wind 3.92 would stay in longer but in no wind situations 3.55 prob would from what ive seen froms posts and such is that 3.55 will have on average better hiway mpg and 3.92 will have better city and towing mpg. also 3.92 have a larger towing capacity . in either axles mpg is usually fairly close cause most people have a combination of hwy \city

 

As Postal indicates there are a number of variables involved in predicting fuel consumption between two different axle ratios. The lower ratio (i.e.: 3.55) will reduce engine revolutions per mile and conversely, use less fuel. In a theoretical condition with no engine load a 3.55 axle ratio will reduce engine fuel consumption over anything higher (3.92, 4.10, etc), and conversely a still lower ratio will reduce fuel consumption even more.

As the engine load increases there is a point where a higher ratio becomes more efficient due to a condition known as throttle pressure.

As the throttle valve in the engine valve body is opened the amount of air (valve body) or air/fuel mix (carburetor) increases (increased throttle pressure) the flow and a corresponding amount of additional fuel is consumed. Therefore, under increasing engine loads a lower throttle pressure induced by a higher axle ratio may offset any benefit realized from reduced engine revolutions per mile.

From a practical standpoint the opposing effects of reduced revolutions versus reduced throttle openings can only be determined directly by the vehicles individual driving cycle. Vehicles that will see a lot more stop-and-go driving, hilly terrain, carrying more load, or lower vehicle speeds will generally see reduced fuel consumption with a higher numerical drive ratio. Vehicles that will being driven on flatter roads, steady speeds, or at higher low-load speeds will generally realize better fuel consumption with a lower numerical drive ratio.

In my own case my 2010 Ram 5.7 with the 3.92 ratio and twenty-inch tires seems to do better than those that I've talked to with the 3.55 ratio and a similar driving style. I have compared gas mileage with a couple of fellow Ram owners with the 3.55 axle and they seem to be getting about 1.0 miles per gallon better than me on interstate highways, but not always.

At 68550 miles my Ram's lifetime average is currently 17.87 MPG. Out of 201 fillups, I have recorded 72 tanks over 20 MPG, with the top 10 all over 22 MPG. I have also recorded 20 tanks under 16 MPG. The highest recorded was 25.006 MPG (373 mile trip on state roads, speeds <60 MPH). The lowest recorded was 10.11 MPG (pulling a 9100 pound tractor and trailer). Mileage is higher in the summer, lower in the winter. My average driving includes fairly flat roads (not mountainous) and speeds 60 MPH or under. I see expressway driving about 15-20% of the time, on average.

I have a neighbor with a conventional cab 5.7 Ram that's driven just around town with lot's of restarts and stop-and-go who says the best he does is 14-15 MPG. Besides the driving cycle, your foot is the largest determinant of fuel consumption.

I hope this helps you.

Best of luck and good wishes.

Regards,
Dusty
2010 Ram Big Horn 1500 Quad Cab 2WD, 5.7 Hemi, 545RFE, 3.92 LSD, dual exhaust, 20” wheels

 

Year.................................... 2009
Model.................................. Ram 1500 CREW CAB Big Horn 4x4 3.55s 20" Rims
Engine................................. Hemi
Highway percent................... 20
City percent......................... 80
Location.............................. Illinois
Octane................................ 92
Hand or computer calc........... Calculator
Miles on truck....................... 91735
MPG.................................... 14-15
Mods................................... replaced muffler with Magnaflow 3in/dual 2 1/4 out
Notes.................................. muffler seemed to add some power and might help out mpg__________________
I drive 7 miles to work one way and turn only 4 times with a full 6 miles stretch in between. It warms up for 5-10 minutes every morning in the driveway due to cold winter weather and being parked outside because its too big for my garage. Still average over 14mpg overall. My 4.7L Dakota only got 16. So I'll take a 2mpg loss to drive a bigger truck that can haul my camper and fit 6 adults comfortably.

 

14-16 driving like a granny

 

I ended up buying the 2012 Ram Outdoorsman with 4.7L, 3.55 LSD, and 20-inch wheels. I've got 1500 miles on it driving city and highway in the DFW area. Fuel computer has but my fuel mileage for 3 tankfuls at 17.1, 18.1, and 18.9 mpg. I drive relaxed and not rushed.

 

Year................................. 2009
Model............................... 1500 Big Horn 4 x 4. 3.92 20 inch wheels
Engine.............................. Hemi
Highway percent................ 80%
City percent...................... 20%
Location........................... NY
Octane............................. 89
Hand or computer calc........ Hand
Miles on truck................... 54K
MPG................................ 17.68
Mods............................... Stock

 

I'm considering buying a 2012 1500 Hemi Quad 3.55 R.E. to pull a 1/2 tonnable 7K lb 27ft 5th wheel. I test drove it and the real time fuel mileage dash display showed 21 +/- .5 mpg at 60 mph. Anybody out there with a similar set up that can tellme what there hwy MPG is?

 

I wouldn,t use a 1500 to pull a 7k 5th wheel.Not enough truck in my opinion.,if you do, get at least the 3.92 rear axle.

 

I've towed a 9100 lb. trailer and tractor twice for about 300 miles in a combination of flat and hilly country and my gas mileage on 89 octane fuel was 11.22 MPG and 10.9 MPG.

Unladened, my summertime mileage around town is around 18 MPG. Highway is around 20. Note my vehicle specs in the signature.

Regards,
Dusty
2010 Ram Big Horn 1500 Quad Cab 2WD, 5.7 Hemi, 545RFE, 3.92 LSD, dual exhaust, 20” wheels

 

Completely jealous of the people saying they're getting ~20mpg.

Year.................................... 2012
Model.................................. Ram 1500 QC 4x4 17" rims
Engine................................. Hemi
Highway percent................... 90+
City percent......................... 10
Location.............................. Sask. Canada
Octane................................ 87
Hand or computer calc........... Calculator
Miles on truck....................... 9500kms (6000 miles)
MPG.................................... 13
Mods................................... Bone stock except for Roll Bak tonneau cover