I have a stock CC 5.7 Sport with 3.55 and I get 20.8 on a 300 mile trip last month. It doesn't seem to matter on the highway mileage which gear you get. I try to tell people to get the gear they need for the job. Even the EPA guide posts the same mileage regardless of the gear, just stay away from the 3.21 with 20's, nothing higher than the 3.55.

 

I have an 09 TRX4 with 3.92 gears. I also have a tonneau cover and a k&n drop in. I have gotten my truck up to 23.5 on the highway on a 40 mile run. Keep in mind this is when it was warmer outside. Now I am only getting about 18 on the highway. The colder it is outside the more power the truck will make but it wont get the same fuel economy. I know someone else with the 3.55 gears and I consistanly get better fuel economy thatn he does especially around town. It makes sense more gear takes less power to get the truck moving, just don't put it to the floor, which by the way is very hard not to do driving a hemi.

 

Truck I drive is 2010 1500 QC ST 4x4 with the 4.7L and 3.92, but it has only a bit more than 2,200 miles on it and basically all of those miles on one long trip.

Got 16 mpg into a strong headwind on the interstate at ~70 mph, 18 mpg with a light tailwind at ~70 mph.

The truck did not have cruise control on that trip, but it has now been added as of yesterday. Might be able to pull slightly better mpg with cruise.

 

Another thing that really helps fuel milaege is a high quality full synthetic oil. Sounds crazy but I switched to Amsoil, and my fuel saver light will stay on longer with the new oil. I run Amsoil in everything I own and always have. I will be switching the diffs over as well as soon as they need it. I had a 03 hemi ram with the same components, fully synthetic, and I was gatting an average of 14.5 with highway of 16, trust me the stuff works.

 

Another Amsoil Dealer ?????

 

www.amsoil.com for all you non believers.

 

Curious if you've actually tried it for yourself and found no difference, or just generally skeptical?

 

We got our 2009 SLT Quad Cab w/ Hemi back in April of 09. It is a 2wd with the 3.21 diff. (We have a bed cover.)

I will definitely agree that having this diff absolutely KILLS acceleration off the line and I started to regret not looking into one with a 3.91.

However, after putting 16K miles on this truck in about 9 months, I have completely changed my mind.

First off, I'd like to note that we use this truck primarily for going skiing and hauling motorcycles out to track days and what not. So 90% of the miles are on the highway (and about 50-60% are on the open highway out in the middle of nowhere and speeds are usually 80-90mph cruising).

With the 3.91 diff, the engine turns about 2K RPM's flat doing 85mph. And better yet, at 70-80mph, when you punch it, it downshifts to 2nd and puts the engine right in it's (max) powerband and it really gets moving. Hits the 105mph governor right around redline.

Just this weekend we took a trip to the condo and our average for the entire trip (which includes both ways and driving around town while up there) was 18.2mpg. Also note that our home is at sea level and the condo is at 8000 feet, and about 5000 of those feet are gained in a pretty short stretch of road (10-15 miles), so it's not like it's 100% flat or anything. And we were not driving slow, steady average of 80mph with 90-95 "sprints" for as long as 20 minutes straight.

On a ~35 mile trip coming home from school once, I averaged 22.3mpg (I was at 23.6 and climbing until I got off the freeway - the 3.21 diff really dies when going stoplight to stoplight).

Before we got the bed cover, trips to condo averaged 15-17mpg. And coming home from the airport once (similar trip to coming home from school) my max was 21mpg.

Around town we get about 12-14, which is terrible (but again, 3.21 diff).

So all in all, depends on what you are going to do.

If I drove this thing as a daily driver I'd definitely want short gearing like 3.91. But for the long highway trips the 3.21 is perfect.

 

Before you can compare mileage, you have to make sure that you mentioned your fuel. I don't mean the octane, it has nothing to do with mileage. I am referring to the alcohol content. If you are running E10, you will get 10-15% less mileage due to the lower BTU content. I get 20.X on the highway with gasoline, but only 18.X with the government's pink pig of ethynol. That crap is such a pack of lies on the ignorant public.

 

A split gear differential would be great then...or GearVendors under/over drive.