307 ring and pinion

[Grandpa54]

I have a 3.73 posi from the factory and want to put in a 3.07. Does any one know if i have to change the posi carrier out? Thanks mark

[Clyde65]

I dont believe you do, however you will need to make certain what series the carrier is, 3 or 4 ( im guessing 4) and you wont be able to use it since 4 series only goes to 3.73.

Good luck. You could call randys ring and pinion to find out. Have your rear end info handy.

[Captainfab]

Yes you will have to change the carrier to a 3 series for 3.42 gears and higher. The 4 series carrier works with 3.73 gears and lower.

[Palf70Step]

When I did my 64, I had an entire 3:07 assembly from a 72. The rear end shop had to do a couple things to match up the 64 axles & all with the 72 stuff. Something about the spline counts are different, which might be the series difference mentioned above. There was also some bearing difference. I can';t remember what else he mentioned.

[Grandpa54]

Thanks guys for the info.

[LVPhotos]

A 3.73 to 3.07 only drops <500 rpm on 28 inch tires. This can be done with taller tire. Look at a rpm chart.

[Grandpa54]

Thanks for that information but the wheels and tires are new. I really like the old 15"

[lumpy63]

Is that your engine-trans set up in picture? If so I would not worry about having 3.73 gears with that overdrive trans, it will work quite nicely I've done it several times

[Palf70Step]

Down here in flat lamd 3.07 with OD sows OK, but you start hitting some hills it does want to kick in and out of OD alot with the 3.07

[Captainfab]

I agree. 3.73 gears is just about perfect for most applications with an OD trans. I would not go any higher than 3.42's, and only if you have tires no taller than say 235/75R15's. Lower rpm's is not always better. The engine needs to operate in it's power range. You currently have a 3.73 gear with a posi, which is a very desirable combo. I would definitely drive it first and see how you like it.

Originally Posted by lumpy63

Is that your engine-trans set up in picture? If so I would not worry about having 3.73 gears with that overdrive trans, it will work quite nicely I've done it several times

[Clyde65]

I had 3.08 before i rebuilt my 12 bolt. I put 3.73's and limited slip/posi in it. The 3.08 were fine at 40mph plus as long as it was flat-ish. Taking off was a little sluggish and very sluggish on medium to large hills, like over passes and such. This is what made my mind up to go 3.73, i also called randys ring and pinion and they were very helpful on the decision.

Good luck!

[Grandpa54]

Originally Posted by captainfab

i agree. 3.73 gears is just about perfect for most applications with an od trans. I would not go any higher than 3.42's, and only if you have tires no taller than say 235/75r15's. Lower rpm's is not always better. The engine needs to operate in it's power range. You currently have a 3.73 gear with a posi, which is a very desirable combo. I would definitely drive it first and see how you like it.

ive put about 1000 miles on it so far, it runs about 2600 t0 2700 rpms at 70mph. My new truck run about 2000 rpms at 70 but it has a 6 speed i believe. I love everything about it just want to get the highway rpms down a little. Maybe a 3.42 would be better.

[LVPhotos]

I run a 3.73 with a 28.5 (295 / 45 / 18 ) with a .67od at 2000rpm or little less at 75. Here is a chart. Tires tend to be cheaper than gear swapping.

[61K10]

what trans do you have and how tall are the tires? I had a 56 Nomad with a 468BB with a 700r o/d with 3.08 gears and they were fine. Now if you never want to take it out of o/d i guess you wouldn't want 3.08/307's

[Grandpa54]

Its a 4l60, My tires are 255 60 15 about 27" tall