rear end questions...

[scuderia]

Looking to swap out the gears on my truck.
Currently has the original 12 bolt non-posi rear end. (suspect it's 4.11's now but haven't pulled the cover to check.) Anyway, since I'm running a granny sm465 I'm figuring that I can go with some pretty tall gearing 3.07 or 3.08 (what's the story with those two? why are there both out there). I know I need a new series 3 carrier as well but what exactly 'am I looking for with regards to years, models etc.
Also, I'm running a 292 at the moment but will eventually be putting in a 327 anyone have thoughts on the gearing choice? I know that there are 3.42's but I'm doing more freeway than I thought and I'd really like to be able to cruise comfortably at 70 with either motor (28" tires on the truck).

Don

[LVPhotos]

Keeps getting asked, here the chart again. I would say you have the typical 3.73 rear

[scuderia]

Originally Posted by LVPhotos

Keeps getting asked, here the chart again. I would say you have the typical 3.73 rear

thanks, appreciate the post but that's really not what I was asking.... easy enough to determine how to get the 70mph cruising RPM via a chart but that doesn't tell me what it's like to pull away from a stop on a hill or tow or many other real world situations. I'm hoping for some feedback from those who have a similar setup and whether they would change anything if they could... more importantly I'd like to know what parts I should be looking for (what parts interchange with my housing, any years that don't work, why a 3.07 and a 3.08, are all 12 bolts the same? etc). Thanks again.

[WOLFMANmike]

Originally Posted by scuderia

thanks, appreciate the post but that's really not what I was asking.... easy enough to determine how to get the 70mph cruising RPM via a chart but that doesn't tell me what it's like to pull away from a stop on a hill or tow or many other real world situations. I'm hoping for some feedback from those who have a similar setup and whether they would change anything if they could... more importantly I'd like to know what parts I should be looking for (what parts interchange with my housing, any years that don't work, why a 3.07 and a 3.08, are all 12 bolts the same? etc). Thanks again.

if you are looking to do any towing with it at all i would stay in the range of 3.90s to 3.42 .. if you go into the 3.08 and higher your truck will find it very hard to get out of its own way while towing ... depending on what your towing it could lug the motor from a dead stop .. now if you plan on just a cruiser and a Hot Rod then your 3.07s or 3.08s will be just fine But your gas MPG in town driving will get worse its a bit of a trade off type of thing ...

[scuderia]

Originally Posted by WOLFMANmike

if you are looking to do any towing with it at all i would stay in the range of 3.90s to 3.42 .. if you go into the 3.08 and higher your truck will find it very hard to get out of its own way while towing ... depending on what your towing it could lug the motor from a dead stop .. now if you plan on just a cruiser and a Hot Rod then your 3.07s or 3.08s will be just fine But your gas MPG in town driving will get worse its a bit of a trade off type of thing ...

Thanks a bunch, that's exactly what I'm looking for.
Don't plan on doing any towing, just hauling a bike or two from time to time and some building materials. MPG, nothing I own gets anything good in that department. More like yards per gallon

[LVPhotos]

I had a 81 c10 with 3.08s a 26.5" tire and 75 was 3000rpm. Your granny gear will help out with tall gears but before you spend all that money look for an overdive. Either motor is still reving high. Taller tire will do the same. I would keep the gears and slow down. Not like my truck now with all new stuff, I am sure you need to freshen up everything like ball joints, steering and brakes before hitting 75.

[scuderia]

Originally Posted by LVPhotos

I had a 81 c10 with 3.08s a 26.5" tire and 75 was 3000rpm. Your granny gear will help out with tall gears but before you spend all that money look for an overdive. Either motor is still reving high. Taller tire will do the same. I would keep the gears and slow down. Not like my truck now with all new stuff, I am sure you need to freshen up everything like ball joints, steering and brakes before hitting 75.

I'm in no hurry but unfortunately everyone else around here is ... average speed outside the city is 70+ and trying to hang around in the slow lane at 55 is a bit hazardous as folks merge on to the freeway at 65+
The good news is the truck is pretty tight overall as it's only just turned 80,000 miles. I've done the driveshaft, u-joints, brakes and shocks so far and will continue to freshen things up as needed.
You mention overdrive, are speaking of something like a gear vendors or switching to T-5? I know the gear vendors are like $3k which is not in the budget.
What should I expect to pay to have a new carrier and gear set installed? I know the used parts are pretty reasonable.
Thanks for the input, much appreciated.

[vince1]

I think the story on the 3.07 vs 3.08 is that 3.07 is for pick ups and the 3.08 came in cars.

I bought a 66 with trashed differential and found one from a 64 with 3.73`s. Teaching my daughter how to drive she asks where is the next gear and I say that's it. She thought that was pretty funny. In the city or on the farm 3.73`s are good but way too slow on the highway.

I found a 71 differential with 3.07`s and like it a lot more. It will do 65 MPH quite nicely.

3.73 with overdrive might be even better combining good low end with good top end.

[LVPhotos]

Any and all od. Look under big ford's for GV od alot of ford guys, my uncle included, had a big nasty 460 and dropped in a Gear Vendor. Long ago I waited 6 months for a t5 , now i saved up and dropped $2500 on New , But a good 200r4 or 400r can be had at less than 2000 with warranty . Remember every time you pass a Bug or Bus there top speed is 65 or less. I had the 9 class offroad car at 75 max desert racing.
Relax take the slow road, most people will give you props and then there are Prius a holes who will speed because they want to save the planet.

[LVPhotos]

Just noticed your in SF. Bullitt had 4 gears and traffic you want gears. Enjoy the ride!

[Captainfab]

With the 3.07 gears you will have to start out in 1st gear instead of 2nd gear as is normal with a SM420.

All truck 12 bolt parts are interchangeable from '63-'82. But beware that the '63 and '64 12 bolts had 17 spline axles and spider gears. the '65-'82's have 30 spline axles and spiders.

[jtrichard]

I have a 75 blazer with a sm465 and 3.07 and 28" its great on flat ground (or down hill in a hurricane LOL) BUT hard to get it rolling and you have to down shift on the slightest hill and mine has a 350 ..... 3.07 is more for an auto trans .....i would stay around 3.42 for all around driving with a stick

[scuderia]

Originally Posted by Captainfab

With the 3.07 gears you will have to start out in 1st gear instead of 2nd gear as is normal with a SM420.

All truck 12 bolt parts are interchangeable from '63-'82. But beware that the '63 and '64 12 bolts had 17 spline axles and spider gears. the '65-'82's have 30 spline axles and spiders.

Thanks Captain!!

[scuderia]

Originally Posted by jtrichard

I have a 75 blazer with a sm465 and 3.07 and 28" its great on flat ground (or down hill in a hurricane LOL) BUT hard to get it rolling and you have to down shift on the slightest hill and mine has a 350 ..... 3.07 is more for an auto trans .....i would stay around 3.42 for all around driving with a stick

How's it around town? Are you using first like a normal trans? Guessing the blazer is about 500lbs heavier than my short bed maybe that would make a difference. Hate to go through all this and find it's still trying to blow itself up on the freeway. Thanks a lot!

[WOLFMANmike]

Originally Posted by scuderia

How's it around town? Are you using first like a normal trans? Guessing the blazer is about 500lbs heavier than my short bed maybe that would make a difference. Hate to go through all this and find it's still trying to blow itself up on the freeway. Thanks a lot!

if you have the funds to do it and you really want to cruise nicely on the freeway and a low RPM the best bag for your buck is to do a trans swap and get a OD trans like a TH200 R4 you will get everything you want and keep you gears improve MPG although your not worried about that ... but you can cruise in the range of 1700 RPM to about 2000 ... this really is the best change up to get what you want IMO .. and will not let you down or be a waste of money

[scuderia]

Originally Posted by WOLFMANmike

if you have the funds to do it and you really want to cruise nicely on the freeway and a low RPM the best bag for your buck is to do a trans swap and get a OD trans like a TH200 R4 you will get everything you want and keep you gears improve MPG although your not worried about that ... but you can cruise in the range of 1700 RPM to about 2000 ... this really is the best change up to get what you want IMO .. and will not let you down or be a waste of money

Thanks Mike I know that is good advice but not willing to give up the manual box as I really enjoy the SM465. Just need lower revs on the freeway.

[66Submarine]

Originally Posted by Captainfab

With the 3.07 gears you will have to start out in 1st gear instead of 2nd gear as is normal with a SM420.
.

??? 3.58 x 3.07 = 10.99 / 2.94 x 3.73 = 10.97. In other words, 2nd gear with the SM420/465 and 3.07's is actually almost identical to the base OE 3 spd/3.73 setup in 1st.

FWIW, my father's truck has a 350 with 3.07 gears and my borrowed 3.11 Saginaw 4 spd (currently running 255/70r15's on the back). We both like the way that combo drives (at least in this area) and 75MPH is just a little over 2,700RPM.

As I have pointed out many times in the past, what is "screaming/about to blow up/comfortable" is also very subjective. My truck has 4.10 gears with a 307/SM420 combo and 235/85r16's, so 2,700RPM is 60MPH for me. It doesn't bother me if I have to run 70MPH @ 3,150RPM to keep up with traffic when I need to.

[LostMy65]

People drive faster than they should.

[Joe Pass]

Originally Posted by scuderia

Looking to swap out the gears on my truck.
Currently has the original 12 bolt non-posi rear end. (suspect it's 4.11's now but haven't pulled the cover to check.) Anyway, since I'm running a granny sm465 I'm figuring that I can go with some pretty tall gearing 3.07 or 3.08 (what's the story with those two? why are there both out there). I know I need a new series 3 carrier as well but what exactly 'am I looking for with regards to years, models etc.
Also, I'm running a 292 at the moment but will eventually be putting in a 327 anyone have thoughts on the gearing choice? I know that there are 3.42's but I'm doing more freeway than I thought and I'd really like to be able to cruise comfortably at 70 with either motor (28" tires on the truck).

Don

I have built many 12 bolt diffs and in my opinion 307 will work well for what you are wanting it to do. 12 bolts came in a variety ratio's and 307 is popular to gain highway speeds on a budget. Gm used the 12 bolts up into 1979-80 and after went to the 10 bolt, essentially a car rear-end with bigger axles.
The 10 bolt rear-end had a 308 ratio and is not interchangeable among the two. Any gm truck 12 bolt internals will work, it is just the housings and axle lengths that changed between vans and trucks and 4wd vehicles. So if you find a donor rear-end with the ratio you want, you will have the gears and carrier to convert yours provided they haven't been damaged in their previous life. Add a bearing kit to complete the reassembly, and you will be in good shape.
I did my 62 using a 65 housing and used some OEM 342 gears with an Eaton series 3 posi and I turn 2900rpm at 65MPH on the freeway with 28" tires.
Good luck on the decisions
Joe

[scuderia]

Originally Posted by Joe Pass

I have built many 12 bolt diffs and in my opinion 307 will work well for what you are wanting it to do. 12 bolts came in a variety ratio's and 307 is popular to gain highway speeds on a budget. Gm used the 12 bolts up into 1979-80 and after went to the 10 bolt, essentially a car rear-end with bigger axles.
The 10 bolt rear-end had a 308 ratio and is not interchangeable among the two. Any gm truck 12 bolt internals will work, it is just the housings and axle lengths that changed between vans and trucks and 4wd vehicles. So if you find a donor rear-end with the ratio you want, you will have the gears and carrier to convert yours provided they haven't been damaged in their previous life. Add a bearing kit to complete the reassembly, and you will be in good shape.
I did my 62 using a 65 housing and used some OEM 342 gears with an Eaton series 3 posi and I turn 2900rpm at 65MPH on the freeway with 28" tires.
Good luck on the decisions
Joe

Thanks Joe, feeling pretty good now that 307's are the right choice for my use. Don

[Shaky]

I put 3.07 gears in my '66, but I also put in a Richmond 5-speed.

My brother had a '68 with a 327, SM465, and 3.73. The rpm drop between gears was pretty large, since the 465 is, in effect, a 3-speed. You may not like the rpm drop between gears with the 3.07…

That said, we drove to Alaska with a 250, 3-on-the-tree, and 3.07.

[scuderia]

Originally Posted by Shaky

I put 3.07 gears in my '66, but I also put in a Richmond 5-speed.

My brother had a '68 with a 327, SM465, and 3.73. The rpm drop between gears was pretty large, since the 465 is, in effect, a 3-speed. You may not like the rpm drop between gears with the 3.07…

That said, we drove to Alaska with a 250, 3-on-the-tree, and 3.07.

Here's my old '65 that I had about 25 years ago, sadly I don't know or remember much about how it was set up. I know it had a 6 in it (probably a 250) and 4 speed. We used to race Willow Springs in SoCal every month and would often haul two bikes down the I5. One time I wasn't driving and woke up to my buddies timing the truck via the mile markers with a stop watch, we were doing 100mph with two bikes and three guys in the cab! Suffice to say I did all the driving after that. Anyway, guess I'm figuring the '65 had some pretty tall gears and I don't remember ever having issue with it.

[bonestock72]

had that exact setup in a 72 4x4 with a 350 sbc. the six has more torque but I live in the mountains about an hour north of tahoe and I found myself downshifting a lot on hills. after messing with these trucks for 30 odd years i believe the 373-411s were for three on the tree, granny gear 4 sp, 342 ratio is for 7oor4, and 307 were for turbo 350-400 three speed auto. hope this helps

[LVPhotos]

1956 Willeys Jeep
289 Ford 2 barrel, 4 speed, 4.56 and 35 in tires top speed 75. All around 55, enough for Mustang trashing in 1/4.
Just saying our aerodynamics suck but some mpg can be had