[Phaeton]

So I picked up a 12 bolt rear with open 3.08 gears. I need to reset up the gears when I install the posi so now would be the time to change the gear ratio. What's your opinions. This will be a street driven only car with the occasional romp. Stay with 3.08 or switch to 3.42.

Thanks for the advice.

1967 Chevelle
454 .030 over
KB Pistons 25.5cc Dome 10.5:1 w/115cc Heads
Aluminum Heads 300cc runners with 115cc chambers
Hyd flat Cam duration @.050 243 intake 257 exhaust
Cam lift is .522 intake and .507 exhaust
Edelbrock Air Gap intake
TH400
3000 Stall Converter
27" tall rear tires.

 

[crazydavey]

I ran the 3:42's for a long time, loved them.....not a gear for going 80 on the freeway but a lot of fun around town.....

 

[mrpaticular]

Find an on line calculator and check the different mph at your present cruise rpm. It could be 8-10 mph lower. Might want to consider this with gas prices high.

 

[MidLife72]

...not a gear for going 80 on the freeway but a lot of fun around town.....[/QUOTE]

Unless you have an overdrive.....:yes:

 

[Ron454]

If you are just adding a posi, the only gear setup that needs to be done is the backlash.
Measure it before you take it apart, and re-set to that measurement.
The pinion can stay in the housing.

Ron

 

[grovey]

342's

 

[stroud6]

I guess this is wear I show how behind the times I am. I see 454 big block with 10.5 to 1 compression. Aluminum heads with 300 cc runners. Turbo 400 with 3000 stall. It would be hard for me to just run 3.42 gears. I would be wanting 3.73 at least. If you are worried about gas milage or how many rpm's you are turning on the freeway look at some type of overdrive or different engine. It just looks like you have a engine built to run don't contradict it with the wrong gears. mark

 

[GRN69CHV]

Based on your combo, go to 3.42's. FWIW, Dropped from 3.73 to 3.42 showed no change at the track with the 469" motor that we pulled. 3.42's will stay in the car for the 540" motor.

 

[Splitter]

The cam manufacturer likely has recommended gearing and converter specs to match the cam. If it were my car, I'd run around 4000 stall with 4.10 gears. Maybe 3.73s if you have to do some highway cruising.

 

[kirkwoodken]

Most guys who have 3.31's won't sell them. That should tell you something. I went 5000 RPM with a 302, 151 cam, 3.08 gears, and 28" tires in a stock-bodied 55 shoebox. About 135 MPH. You should have no trouble doing that with 150 more CI and hundreds of HP more.

Did 6600 RPM with 68 Z/28, 3.73, 26" tires. About the same speed. With 2.2 and 3.73"s, the car would go 70 MPH in first gear. Think about what the same car would run in 1st with a glide.

Figure on 2500 RPM, or below for good hyway cruise RPM. To get good mileage, you want to work the engine. If you cruise at a low throttle opening, you will not be running at high compression. If your overall gearing is in the low 3's or high two's, you will need more throttle opening to move the car. Although the throttle is farther open, the engine is running at lower RPM and higher compression, which makes mileage go up.

Don't gear too high for a car that will be hyway driven. At $4 a gallon, you don't want to change gears twice.

 

[grovey]

i sold the original 331's outta mine, way to cheap i might add.... sure would be nice to have them now.

 

[Yellomalibu]

So I picked up a 12 bolt rear with open 3.08 gears. I need to reset up the gears when I install the posi so now would be the time to change the gear ratio. What's your opinions. This will be a street driven only car with the occasional romp. Stay with 3.08 or switch to 3.42.

Thanks for the advice.

1967 Chevelle
454 .030 over
KB Pistons 25.5cc Dome 10.5:1 w/115cc Heads
Aluminum Heads 300cc runners with 115cc chambers
Hyd flat Cam duration @.050 243 intake 257 exhaust
Cam lift is .522 intake and .507 exhaust
Edelbrock Air Gap intake
TH400
3000 Stall Converter
27" tall rear tires.

To put the posi unit in, all you have to do is swap ring gears and set backlash - pretty simple; not rocket science.

To change gear sets, you have to set the pinion depth and set the backlash.


I vote for keeping the 3.08's in it. If you don't like them, change them later. Being a street only car, you will likely enjoy the lowered rpm's they provide.

... but if you do install the 3.42's first - keep the 3.08's in case you change your mind.

A buddy's dad has a 12 bolt with an open differential he bought from a junk yard. He doesn't know what gear it has. He is going to give it to me if I do some work for him he doesn't want to do (painting the metal roof on the second story of his house).
I'm hoping it has 3.08 gears. (and I have a cam with more duration and more converter than you do)