Trans Gear/ Rear Ratio/ Tire Height/ MPH = 60 65 70 75

5th 3.55 32.2 RPM = 1470 1590 1710 1835

5th 3.55 33.2 RPM = 1420 1540 1670 1780


5th 3.92 32.2 RPM = 1620 1760 1890 2025

5th 3.92 33.2 RPM = 1570 1700 1835 1965

 

4x4 hemi with 3.92, I get 14/15 around town, 17/18 highway 75mph with cruise in, In fl no hills, but if the wind is cranking you will notice a big change depending on head or tail wind. I think these are respectable numbers for a truck this size and power. If I tow a double axel trailer with 3 atv's and a bunch of other stuff I get 10/11, truck seems to handle towing no problem. My sons f150 with the 5.4 does worse mpg in all aspects and it's a wimp.

 

3.92 will get better mileage than 3.55 with 20's if you drive alot in town or haul/tow.
It is all about torque. Use the search button on this forum. I wrote a detailed explaination on this topic several months ago. Stay away from the 3.21 axle with 20's unless you want a dog and a lugging engine. The 3.92 is limited slip and the 3.55 is open. I have the 3.55 on my 09 sport CC because it was a great deal as a demo, but I would never order or buy one of the lot that was not 3.92 or 4.10 on these trucks unless you are wanting the 17 inch tires.

 

If no head wind,good fuel ,20 mpg at 72 mph.My truck gets a tad better mileage at 72/75 mph then at 70 mph. 3.92 gear.Most of the time its showing in the 17 mpg range mixed driving.I'm somewhat rural.Not close in Atlanta.

 

How do you disable the MDS since our truck is mostly city driven I don't think I need it going in and out I can just imagine something wearing out.

 

I have the 3.92 rear end and am very happy with it. The salesman that I bought it through is a friend and the one thing that he totally reccommended was the 3.92 gear ratio. At 70 mph last fall going north for hunting loaded with gear, beer and a 700 lb quad on the back I got 22 mpg (imperial) and 23.5 on the way back minus the quad. I am happy with that. Previous F-150's were only at 19-20 mpg imp. max. The Ram with that rear end even stays in MDS for stretches when the road flattens out and the wind is not bad at 70mph. Also, when I hit some very hilly country farther along it did not once downshift climbing hills.

I have only pulled a fairly light trailer once and was really dissappointed with the way the truck was performing. Halfway through the trip I switched into tow/haul mode and the difference was night and day and the truck pulled like a champ and better than my previous F-150 with the same trailer and load behind it.

MPG is really dependant upon your driving habits. I have found at different points where the truck likes to go into MDS and then try to stay in that range when traffic allows. If you like to drive hard and heavy you will pay at the pump no matter what you drive.

 

Checked today.... it's about 1600 rpms at 60mph.

 

I generated an exell spread sheet for my truck. All I have to do is plug in the speed and it figurtes out the rpm for each gear set. I have the 3.21 with the 26570R17 wheels (31.7" tall). Just ignore the RPMs for the first gear.
At 60 MPH

Sorry folks but I am having difficulty getting the graphics to work - I will have to try another time.

 

I have a '09 with the 3.21 and 20 inch wheels. It's the fastest vehicle I have ever had. It's not a dog. It runs great! I have blown off a few doors with it. Mileage is good.
I have always had Ford but I like this much better.
My last Dodge was a '87 318.
Jack

 

We have a 09 Hemi Laramie CC 4x4 with 20" wheels and 3.92 rear end. The bed is covered by a matching Leer cap. Other than that it's all stock. Just over 9500 miles of combined driving pulling a 6000 lbs trailer, city and limited highway driving. The worst mileage 7.2 mpg pulling the trailer 70-75 mph 10-20 mph windy day. Best 18.6 mpg driving without trailer highway and back roads and just taking it easy. I don't know how people are getting 20+ mpg. Overall for everything to date 12.2 mpg. All hand calculated.
May not be the best mpg vehicle out there. But it's been the most all around enjoyable I've ever owned.