I carry weight in the back when the roads are bad and no need for winter tires or studs. Growing up, no one had 4x4's and yet we all got around somehow

 

X2 on highway mileage. No way driving at 2200-2500rpms will be cheaper than stock unless you've got some huge tires maybe.

 

As the owner of three trucks that I've re-geared I can tell you that in each case my highway mileage decreased, although not a significant amount and with the case of my 3rd Gen it's just down a fraction because of basically having two over-drive gears.
Now towing I can agree because my truck doesn't search for gears nearly as much as with my original 3.92 gears after going to 35" tires (which basically gave me an effective ratio of about 3.6:1)...

 

I have a 02 1500 QC 4.7 4x4...Im running 4.56 gears..An I love them..Im only running 33's..My truck runs a lot better and Im very happy..I get better mpg until I do well over 70mph..If I do 80mph the I get like 10mpg..At 80mph Im turning 2800 rpm's..Not bad at all..I think you will be much happier with 4.56's an I bet you will see a increase in mpg all the way around with those 35's....An that's wht size I will be running as soon as these 33's run down..I don't think you will be happy with 4.10's....Not with a 35" tire..

 

It's not like the EPA takes an example of each vehicle and extensively tests them for mileage.. instead, they use a formula based on engine RPM and gearing (including trans, axle, and stock tire height which equals overall drive ratio)..

RPM's, forward speed, and overall drive ratio are a mathematical equation that is etched in stone- if you know two, the third can be figured; unless something is amiss such as a slipping transmission..

it's this equation the EPA is concerned with- and the reason manufacturers use higher (lower numerical) gears, especially on their gas guzzlers.. to hide the true consumption..

there is a 'overall fleet' economy number manufacturers have to meet, mandated by legislation, which will result in a fine if it is not met.. -- thank you tree huggers...

an engine 'lugging', such as these heavy trucks, will suck down more fuel than one pulling comfortably (as registered on a vacuum signal, for one indicator).. gears/leverage allow the power produced by an engine to be used more effectively and efficiently, IN REALITY, NOT ON PAPER... the pencil heads at the EPA, and people who never use their trucks like trucks will argue this, but it is true. Lower gears (within reason) get better mileage unless you are stretching its legs out on the highway for extended runs..

I've often lamented that 4.30:1, if available, would be about perfect- especially pushing 35" tires (and on the highway).. 4.56:1 is available, and the ones I choose.. 4.10:1 would be better if I spent a lot of time on the highway, but not so good while hauling and/or pulling stuff around stop and goes- (you know, like a truck ought to be used)... :-)

 

It's my personal opinion that an effective ratio of around 4.10 is perfect for these trucks and remember by EFFECTIVE ratio I'm talking about the actual gear ratio adjusted for tire size. So 4.56 gears adjusted for 35" of tire diameter is actually just a tad under 4.10:1.

3.92 - 4.10 is damn good for stock'ish size tires up to about 33", but once you get up into 35" and up tires if you're gonna re-gear I don't think you should be looking at anything other than 4.56s unless you never tow/haul and live mostly on the interstate.

If you're worried about spinning out on slippery conditions then there is always the TruTrac or Auburn LSD options...

 

Even if you ran on the highway all the time with 35's I'd say 4.56's...Im running 33's with 4.56's.I drive 70miles round trip to work every day and 90 percent of that is highway...I don't have to fight to maintain speed anymore..Its so much better..I almost did 4.10's..Im so glad I didn't

 

Well I was going to buy the Detroit trutrac at the same time, and the install kit with the bearings and everything for front and back, figured i'm at 180,000 and so I don't want to put new gears in and have old parts break them when they go.

After prerunner kit, a stage 2 sharadon valve body upgrade, larger transmission cooler, I plan on getting an engine rebuilt to sustain 14 or so psi (currently I run 7 from supercharger).

My biggest concern is that I already spin out easy if its even a little wet with all the power, even at 3.56, even on dry if I start at WOT I spin out a bit. I plan on making it around 800hp/800tq by the time I'm done as well with the fiber glass bedsides I'm going to have even less weight in the rear. I don't want to spin out even on dry every time I go WOT.

Also just want to add if it makes difference, I don't really do any hauling/towing its basically just toy.

A question for those using 4.56's what is your highway rpm for like 105-110 and with what size of tire?

 

^Something like this might help you

http://www.ringpinion.com/calculators/calc_rpm.aspx

 

With the extre OD gear of the 5-speed auto, running stock 275/60r20 (33" tall) tires and a 3.92 gear, my rpm at 70mph is about 1950rpm. With 4.56, it would be around 2200rpm. Not much of an increase at all so highway mpgs will drop very little.

According to the chart in the post above^^ 4.56 gear/33" tires/70mph/0.67od = 2178rpm