I would like to replace my 3.56 rear end (99, 2500, 2X4) with a 3.10 or so. I asked this question and also did a bunch of checking a year ago but never came up with a solution. Just asking again to see if anyone can come up with anything.

 

The only way to get less gear than a 3.54 would be to swap in a D70 U housing which has an offset pinion bore compared to a standard D70 which allows for a pinion with a higher tooth count. The D70 U uses it's own ring and pinion sets that aren't interchangeable with a standard D70. There is no other way to get less gear in a D70.

A more viable option might be to run a larger diameter tire...

 

they make a 3.31 gear for D80's

I wish I could get 3.25-3.30 gearsets for my truck and step down to the factory size tire

 

I think I'd just get a overdrive unit,that would give you both gears..
My .02

 

Isn't that the D71? I thought the D70U was like the D70, but with a baby smooth underside. Just like the D60 and D61, which are available in the first gens, I believe.

 

CTD Nut: How would I go about getting the parts you refer to? Would it require me changing out my entire axle and rear end? Do you have any idea of an approximate cost? Also the idea of an overdrive is a good one and I've considered it. My purpose in all this is to reduce my RPM to improve fuel economy.

 

Do a search on here about those overdrive units. Most of I've read on here is that they only give a marginal improvement, if any. Plus that most are installed post t-case (if 4x4), limits its use to 2wd only. And the price!

 

I can't imagine wanting... or gaining mileage from a taller gear than the .68 overdrive unit you have with your 3.55 rear end. Especially at or over gross weight with your flat bed and camper?

JMO

 

I don't pull anything... I'd like to go to a shorter tire, but keep the revs down on the highway. Right now, cruising at 82mph, I'm right at about 2000.

I'd like to be able to do 85-87 at 1800rpm

 

rjohnson: I've done a number of tests to determine the status of RPM on fuel economy with hand calculated MPG. Here are the results of one test. I ran the truck at 55 mph in drive but out of overdrive on level ground and was running at 2200 RPM and got 22.5 mpg. I ran the same road in overdrive and was running at 1500 RPM and got 27 mpg. I have done a number of other runs at various other speeds and it all points to considerable better mpg if I get the rpm down. Another concern with an aftermarket overdrive unit is there is some effeciency loss going thru extra gears.

 

I can't argue with your testing.

Are you saying you would like to go faster at 1500 rpm, or you wish to run 55 mph at a lower rpm?

Not an expert by any means, but my brain suspects your 27 mpg may be 'the wall' and as good as it gets.
I would enjoy being proven wrong.

Good luck - RJ

 

man, 55mph is what people drive in town! I'd just like to keep the revs down a little at highway speed... not so low that it's lugging it, but something to keep the resonance down and maybe get a little more mpg... you obviously run into diminishing returns by going with bigger/heavier tires to keep the revs down

 

There's been a lot of testing on semi trucks about what you're trying to do. Basically their conclusion was the best way to get better mileage was to slow down. It's called "gear fast, run slow".
You get into too much wind resistance at higher speeds, which negates any lower rpm advantage. I don't remember the exact numbers, but there's a formula about how much more hp is required, for every extra mph.
The faster you go, the more power required to overcome wind resistance, making the motor work harder, and use more fuel.
Your best bet would be to go with fully synthetic oils, free flowing intake and exhaust, keep the tire pressures up, work with aerodynamics, and you could try bigger tires, with a summer tread design, but you need to get as low as possible also.

 

aerodynamics is the biggest killer... I can't remember the formula, but it's something like as speed doubles, the power required quadruples or something nuts like that

 

I did the same MPG test at higher speeds up to 75 mph and got the same results, better mpg at if I lowered my RPM's.