results from a rear end swap

[shrtbx]

Hey guys,
Finally got my truck back from oklahoma. I had to leave it up there after i made the drive from los angeles in december for a family issue. Truck ran great going cross country.

Only thing that sort of bothered me was the rearend. I have read on here that the 700r4 with 3:08s really arent really that efficient as far as the powerband goes. I average 16 mpg on the highway but i felt like the engine was lugging. I had to keep on the gas alot to keep it up to speed. In low mountain ranges i had to dang near keep it open just to get up them. I really think i can squeeze another 1 or 2 mpg if i upgrade to a 3:42 and get a lil more get up on inclines. It just seems like my engine is working harder than it needs to. Anyone have any actual results from switching out their gears. I would like to hear them.

I run a stock 350 with edelbrock 1406 carb and edelbrock performer manifold. Hooker headers etc etc

[STOCKISH]

I have had good results running a 700r4 with 3.73 gears. Great all around combination. I am sure 3.42 gears would be ok too.

[JHILDE]

I went from 2.73's to 3.73's with the 700r4. Got better mileage, don't remember exactly how much but it was noticeable. Picked up some time at the drag strip too.

JHILDE

[shrtbx]

Thanks for the fast response fellas.
. I figure i will get better mileage on the highway and on the street it might drop. I can live with that though.

[Pyrotechnic]

Not exactly what you are looking for but somewhat related - When I got my truck it had an open 3.42 rear end and with the stock engine in it, it ran really well. I had a 3.08 posi rear from my old truck so I decided to swap it in. After that the gas mileage went down 2-3 MPG, and it was just sluggish in general. Also ran hotter too.

Eventually I did a cam swap that really boosted the low end torque of the motor and that got back some of the driveability and gas mileage. Just an example that shows how gearing has to be matched to the torque curve of the motor. Honestly, it still has the feeling like it's never in the right gear. Something about being able to do 60 MPH in first gear isn't right either.

A 3.42 gear will work well for you, assuming you are running a 28" tall tire.

[shrtbx]

its pretty close to 28 inches. I have a pretty nice combo as far as my cam and manifold go. I really think the gears will bring everything together. I will proboably stick in an MSD box i have laying out in my garage as well. I suppose that wont hurt either.

[nlped]

I went from a 2.73 to a 3.42 with a 700r4 and my truck is a whole different animal. It's more responsive and the mileage went up 2-3 mpg on the interstate, but is about the same in town.

[shrtbx]

what kind of mileage did you average before the swap NLPED.

[pcmcobra]

i went from 2.73 to 3.73 w/ a 4l80 and it's a different beast now.
mpg? no idea and don't care, too much fun!

[nlped]

14-15 before and have gotten upwards of 18 mpg average on a tank.

[shrtbx]

yeah thats the number im shootin for...Good stuff guys. thanks

Whats the size of a 1986 rear end. 8.5?

[spaseghost]

i believe its an 8.5" 10 bolt, totally not sure about splines though.

[nlped]

If it is the stock rearend it is an 8.5" 10 bolt w/ 28 spline axles. I don't really know why this is relavent, but that's the answer to the previous post.

[silvereagle07]

Originally Posted by nlped

If it is the stock rearend it is an 8.5" 10 bolt w/ 28 spline axles. I don't really know why this is relavent, but that's the answer to the previous post.

I think he is trying to get info to buy the correct gear for the differential. I have run 3.08,3.42 and a 3.73 in my truck with the stock 305. I like the 3.42 overall. It has a good take off the line and decent highway with the 700r4.

[nlped]

I think you are right, Jeramy. Just for future reference, the spline count doesn't matter when it comes to the gear selection, just the ring gear size...i.e. 8.5" 10 bolt.

[Kaysenmc]

I will have your answers in a few days!! I have an 87 swb 2wd with a 700r4 and 2.73 10 bolt and am about to swap in a 3.42 geared 12 bolt out of an 85 lwb I parted out some time ago. Just finished installing new seals and checking everything out, now just gotta wait for the rain to slack off. but I will give my results asap

[3r!c84]

Originally Posted by silvereagle07

I think he is trying to get info to buy the correct gear for the differential. I have run 3.08,3.42 and a 3.73 in my truck with the stock 305. I like the 3.42 overall. It has a good take off the line and decent highway with the 700r4.

yea i got a 3.42 in my 305 th350
wish i has a OD

[Tony_SS]

Anyone try 4.11's with a 700r4? Think they'd be too steep?

[slugish]

Iran a 3:73 in a turbo 350 --too steep (for me)

think a 4:11 in a 700r4 would be about the same rpm's at any given speed .

4:11 would be great for a drag truck > not so great to cruise

[Justins76]

depends on your tire size

[shrtbx]

I have a brand new edelbrock model quadrajet that im thinking about experimenting with as well. I might be spinning my wheels a bit but i just really want to see how much i mpg i can squeeze out without the huge sacrafice of power. I hear that a well tuned quad just cant be beat for the street. I am really happy with the edelbrock 1406 i have but i want to see what kind of luck i have with the quad and its smaller primaries.

I think that with tuned engine and the right combo of parts, right trans gears etc etc, we can have both decent mpg and power. I believe in fuel injection and its advantages but i think 18-19 mpg out of a carb and above average power is something that can be obtained.

Hey call me a dreamer...

[f.monroe]

Originally Posted by Tony_SS

Anyone try 4.11's with a 700r4? Think they'd be too steep?

A friend had a 93 stepside with a 700 and 3.42's stock . It ran a 295/50/15 on the rear (this was the 90's) and it ran good . He swapped to 4.11's and it was too much in 1st gear . He went back to the 3.42's and was much happier .
Frank

[shrtbx]

most guys i know with 4.11s in their cars run them only with manual 5 or 6 speeds. Thats on an everyday driver of coarse.

[glock35ipsc]

I personally think 3.42's with a 28" tire (like a 275/60-15 BFG) is about perfect.

3.08 = 2250rpm @ 87mph
3.42 = 2250rpm @ 78mph
3.73 = 2250rpm @ 72mph
4.11 = 2250rpm @ 65mph

[68 TT]

Originally Posted by Tony_SS

Anyone try 4.11's with a 700r4? Think they'd be too steep?

I went from 3.42's to 4.10's in my 90 Z71 and it was a huge improvement but it was running 33x12.50's. 70 mph was just over 2000 rpm.

With only 28" tires the 4.10's would be OK with the OD of the 700R4. I ran these in my Malibu with a T-350 and 28" tires and 60 mph was right at 3000 rpm. The 30% OD of the 700R4 would drop that to about 2000 rpm.

In a car with shorter tires 4.10's are too much with the 700R4. Most street performance cars with the 700R4 run 3.73's but they are also running 25 to 26" tall tires.