swapping 3.73 open for 3.42 posi

[gottattooz]

I currently have a 68 short/wide with a 4.8l/4L80e LS3 swap with a 3.73 open rear gear. I'm getting 15-17 mpg currently, that I'm very happy with.

I also have a 65 Buick skylark wagon with a 5.3l/4L60e LS3 swap with a 3.42 posi that's getting 20-22 mpg.

Now I'm jealous of the mileage I'm getting from the wagon. I've never done any swapping of the differential gears/guts. Just looking for ideas as far as swapping out a complete rearend with the 3.42 posi, buy a posi setup and have someone help me put it in, or just be happy with what i have and leave well enough alone.

-Josh

[crazy longhorn]

Just a thought...3.73, to 3.42 is not a big change? You might look at tire sizing on the rear.....run a taller tire that will effectively change the gear. On your truck, you will need to swap the 4 series carrier to a 3 series, to run the 3.42 gear. 3 series, is 2.76- 3.42 on the truck 12 bolt....4 series, is 3.73 & down. If you have the special wheel/tire combo, what I suggest wont work for you...just a thought Longhorn

[b454rat]

Granted the wagon is a heavy car, but I think the truck is heavier. You might want little more gear for a truck...but do what 'horn said, put bigger tires on it and see what/how it does.....

[gottattooz]

I had 275/60R15's on the truck. I swapped rims and went with 255/70R15's to get better mileage, but I don't like the look of the taller sidewall on the rear tires. I'd like to go back to a 60 series sidewall profile.

Knowing that I can't just swap gears in the housing means I have to find a complete rear end is actually easier to swallow for me, since I don't have to mess with the internals and risk eating bearings and gears if I set it up incorrectly.

The difference in factory vehicle weight is within 100 lbs of each other (33xx for the wagon, 34xx for the truck. I looked it up once).

I don't know much about differentials, so thanks for the input.

-Josh

[nonstop]

Stay with what you have and enjoy it. I could be wrong here, but worth pointing out anyway. Unless I'm mistaken, 3.42 was not a factory option then. So, you will either have to find a coil spring 12 bolt someone else had as a project and is getting rid of, or pay someone to set yours up with gears.

Second, the truck is pushed by a 4.8 compared to the 5.3 in the car. It probably already takes more throttle to get it going, especially around town. 3.42s will likely make the engine have to work harder to get to speed. You might actually be losing mileage going to higher gears if you drive a lot around town.

[Captainfab]

I have to agree that the 4.8 may be working just a little harder plus being bolted up to a 4L80E likely isn't helping. It takes more power to turn a 4L80E versus a 4L60E.

Even though the vehicles are close in weight, the aerodynamics are worse with the truck and the engine and trans combo's aren't quite an apples to apples comparison.

[BigDan3131]

Well you could just get one size larger rims out back and go back to the 60 series tires and it really isn't that noticeable between a 15 and 16 incher.

[Acorn]

What type of driving do you do? City/hi way/ both?

Do you already have a donor rear ended?

At 65 mph, you would only be lowering your engine RPM by about 160.

Jumping way down to a 3.07 would drop your rpm down about 350 @65. But it would be a dog accelerating from a stop.(my opinion) And might actually bog your engine down at highway speeds and decreasing your mpg.

Seems like a lot of work for little gain.

The real question should be how bad do you want a posi?
Just my $.02

[Cun-tree]

I may be wronge but I dont imagine that would give you any better mpg with the small change. I only got 17-18mpg in a 2000 chevy with 308 gears. My brothers 99 4.8 swb with 308 gears I think gets 17-19mpg
Thats alot of fun factor lost, for not much gain.

Plus it will take a lot of gas saved to equal the 800 dollas for parts and labor.

[gottattooz]

Years ago, i swapped a 3.42 for a 3.08 in a S10 with a V8, and it got worse mileage. As mentioned by acorn, it was a dog off the line and lugged in overdrive.

I'll stick to what I've got. Thank you for all of your input and advice.

-Josh