About differentials

[BJedi76]

I have a 1986 GMC 4x4 Sierra classic 1500 SWB.

I live in Alaska so, half of the year I’m driving on ice and the other half I am driving on pavement and earth.

I don’t haul things or tow things.

my question is; What is the highest ratio rear end for highway mileage that I can get with Posi/limited slip front & rear?

 

[SquareRoot]

You can go down to 3:08 at least, but with a 305 and OD the truck might not have enough power to get out of its own way?

 

[shiftpro]

We can be of more help with more info please.. tire size for example. Engine and transmission. Do you have a tach? If so, what rpm at 60 mph?

 

[Grit dog]

You've been after mpgs since you got this truck. You said you don't tow or haul.
First, what gears are in it? Second, take the cost of the work to rebuild both diffs, all in, including ancillary stuff and compare it to the theoretical mileage gain, if any, in gears and see how many miles you need to drive to make this a viable project.
A 2mpg gain in normal mileage for that truck will break even in about 50,000miles if you include labor and all in for new gears. Unless it's deeper than 3.73s, you have overdrive already, so I don't think you'll see much gain.

 

[C10MixMaster]

You must be registered for see images attach

 

[Matt69olds]

You've been after mpgs since you got this truck. You said you don't tow or haul.
First, what gears are in it? Second, take the cost of the work to rebuild both diffs, all in, including ancillary stuff and compare it to the theoretical mileage gain, if any, in gears and see how many miles you need to drive to make this a viable project.
A 2mpg gain in normal mileage for that truck will break even in about 50,000miles if you include labor and all in for new gears. Unless it's deeper than 3.73s, you have overdrive already, so I don't think you'll see much gain.

Agreed. You can buy ALOT of gas for what a gear swap will cost.

As far as limited slip differentials go, there are many options. A limited slip differential in the front can cause funny handling, keep that in mind.

 

[Grit dog]

Besides, 305 trucks that “look” like the OPs generally came with pretty tall gears.
OP just needs to see what’s in it before thinking about swapping.

 

[Bextreme04]

We have 3.08's on our newer suburban. They are good for the highway and I get about 20mpg on freeway driving. They are terrible for pulling any kind of load or driving around town though. It's on my list to change them out soon to 4.10's so that it does a bit better around town and it should really help when pulling a load around Oregon and the PNW where we have lots of mountain passes and steep pulls. Since it has a 5.3L LS and the double overdrive of the 6L80E, it shouldn't make a huge difference on highway mileage either.

 

[68post]

All good advice.

My friends '87 V10 had the 4.3 6 cyl and the 4 speed manual OD trans with 33" tires, it was a disaster in OD unless you were over 70/75 mph - but it wouldn't maintain that speed and you'd be forced to downshift. Meaning - be careful with your gearing, if you have a stock half ton - leave the gearing alone.

I think you'll need to look elsewhere for mileage !

 

[Ricko1966]

I.ve tried to stay out of this but, I finally got sucked in. 90 percent of 8.5 gear carriers are going to be 3 series with those you can run 2.73 - 5.38 gear sets. Posi and limited slip in a truck, on ice, your going to end up with a truck that won't go won't steer and can't stop. My 2 cents spend the money on good tires and gasoline.

 

[80BrownK10]

Agreed. You can buy ALOT of gas for what a gear swap will cost.

As far as limited slip differentials go, there are many options. A limited slip differential in the front can cause funny handling, keep that in mind.

I looked into a gear swap years ago for better MPG. I can to this exact conclusion. Especially in my case where it doesn't get driven more than a few thousand miles a year at most. Now if it hot a little better I would drive it more but. It's an old truck. There called squares. They get the common MPG for the era. Move forward 20 years they only do 3 to 5 mog better at best.

Take that money you want to spend on gears and what your spending on gas and buy a beater Toyota Corolla or in your case a Subaru to commute with. I know it doesn't look cool and you have insurance to buy but unless you have a DUI and that SR22 insurance or whatever they call it it really won't be that bad, especially since you can then lower the miles per year driven on the heavy old truck.

 

[RecklessWOT]

I looked into a gear swap years ago for better MPG. I can to this exact conclusion. Especially in my case where it doesn't get driven more than a few thousand miles a year at most. Now if it hot a little better I would drive it more but. It's an old truck. There called squares. They get the common MPG for the era. Move forward 20 years they only do 3 to 5 mog better at best.

Take that money you want to spend on gears and what your spending on gas and buy a beater Toyota Corolla or in your case a Subaru to commute with. I know it doesn't look cool and you have insurance to buy but unless you have a DUI and that SR22 insurance or whatever they call it it really won't be that bad, especially since you can then lower the miles per year driven on the heavy old truck.

While I totally understand what you're saying, two things to consider- depending on what part of Alaska we're talking, not having a tall truck with 4x4 might not even be an option (most of the state is backwoods AF, dude might be off grid for all we know), and FWIW Subarus aren't cheap (even for old beaters for some reason they hold their value really well) and they get **** gas mileage as well. I have owned several and only ever got over 20mpg on super long highway trips on mostly flat ground, the same sort of trips that my OD square gets 16+mpg.

I get the point you're making, but most likely OP is better off just sticking with his truck and paying for gas. Even with a cheap beater Corolla or Civic, you still have to pay for the car up front, pay to register it, and pay insurance. Even with double the gas mileage, unless he gets rid of the square (which doesn't seem likely) it will take YEARS to pay for itself, as would the gears (at least you don't have to insure those lol.

You could look into getting a little S10 or Toyota Pickup or something, but even those IME are sub 20mpg vehicles, and then you have even less power and tow capacity than the 305...

In this situation, there may be no magical answer to save money. Living in AK isn't cheap or easy from what I hear. Hell, I dream of living there but I know it wouldn't be easy

 

[Rusty Nail]

Let's get real folks...all of this dude's problems could be solved IF he would simply add vortex heads.

Amirite?

 

[Salty Crusty]

Nope, you am wrong. Unless, of course, the entire fuel injection system is also added.
Vortec heads have NO heat crossover and it will be a turd to try to drive when cold, particularly in the great white north. I had a buddy that went through the same thing in Colorado. Pulled the Vortec heads off and added a pair with a heat crossover and his cold driving problems disappeared.
They're fine in their OEM configuration because the fuel injection system runs the engine rich until it gets warmed up a little bit.

 

[skysurfer]

Nope, you am wrong.

Me thinks you missed the tongue-in-cheek humor of Rusty’s post.