question about c10 axle with 4.10

[70chevrolet]

Hoping someone with more experience on this subject will chime in.
I recently built put together a 300hp sbc for my 1965 c-10 short bed. I installed it with a muncie M20 and my tire size is about 27" radius. I have a 12 bolt open diff with 4.10 gears that was original to the truck.
My problem is that the 4.10's are way too tall and 1st gear will tear your face off and 4th gear is good to about 50 mph. I want to swap to a set of 3.08's so I can enjoy this thing...(I defineately want 3.08 or close, I have considered 3.73, 3.55, 3.42, etc, etc) I didn't intend to build a tire shredder, I wanted a cruiser. Anyone know what years axles are interchangeable with these trucks and also if any other make and model gmc axles were interchangeable? I am thinking It would be more cost effective for me to just swap out a whole axle than to pay someone to re gear my axle. Because you know once I'm in there, I have to go with new bearings, gears, A sweet Eaton posi. not to mention the cost to set it all up.
I know chp makes kits to cut the brackets off a axle of similar length and fit them into this truck, but don't feel comfortable doing that welding myself.. I need a direct swapper.. I can swap axle shafts and/or brakes if need be.
Thanks in advace
70 €hevy

[Captainfab]

The only direct bolt in rear differential swap for your '65 will be a '65- early '70 12 bolt from a C10. And those will all have the 6 lug wheel stud pattern. The most popular gear in these are 3.73's, which aren't a big step up from 4.10's but a little. You might get lucky and find a 3.08 geared rear differential, but I haven't seen very many. You would stand a better chance of finding a 3.08 or a 3.42 geared rear differential if you expand your search up thru '87. Of course there will be cutting and welding required for the installation of those. If you need a 6 lug, look at the K series trucks, or K5 Blazers. The 5 lug rears will of course be in the C series trucks. Also keep in mind that the '71-'87 rear differentials are 1.5" wider overall than your '65 rear differential. Another thing to consider is going to a taller tire.

BTW 4.10 gears aren't 'tall' gears, they are 'short' or low gears. A 3.08 would be a tall gear.

[Heater63]

Your right, you would end up totally rebuilding the rear end. To make that big a gear change you will have to change the diff too; 3:08 uses a different series carrier!. I would suggest a new posi unit (look at the TruTrac helical gear one too) with new ring and pinion, and new 31 spine axles. About $1K for those parts, a couple hundred more for someone to set it up (around here), plus seals and bearings, maybe a new yoke.

Maybe leave the rear alone and run an overdrive transmission?
Use an online gear calculator...you might find your current setup would be OK with overdrive and another inch taller tire too.

[blackedoutharley]

$700 in parts including new gears, posi and all the bearings from Randy's Ring & Pinion.

www.ringpinion.com

At least you will know what you have and one less thing to worry about.

[70chevrolet]

well that really limits my options there. I guess I got to cough up a grand to play. Thanks for the information. I was kind of thinking that this would be the case. I will have to sit on it for a while as $1000 bills dont grow on trees. and 3.08 axles just dont come around that much.
And for sure I do mean 4.10 are short or low gears, where as 3.08 would be tall or high gears.

[taank]

I have 3:73s with a 28 inch tire it's around 3:42 if I recall
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[Clyde65]

I dont think you would be happy with 3.08's, If it were me, which it isnt, I would strongly consider going to 3.42's. but that is me.

[blown240]

Here is a so-so comparison for you:

In my 51 Chevy car I have a 350 that is close to yours in power, with a th350 trans. I have a 2.43 posi 10 bolt rear end, and at 70mph I am at about 2100 rpms. Thats with a 27 inch tire. Its a great cruiser, and there is still just enough grunt to light up the tires for about 30 feet.

[mikie 1954]

Two things first-blown 240 did you mean 342's? Second- 70chevrolet stated that he had 27" radias tires , that would be 42" tall, i think you ment 27" diameter? Not trying to start any thing here just want everyone on the same page.

[awbrown]

According to my chart a 27 inch diameter tire with 4.10 gears will trurn 2806 rpm .. 3.08 will be 2108.. these calculatiuons are for 55 mph.. My truck has the 3.08 , listed in the options as the economy gear...

[blown240]

Originally Posted by mikie 1954

Two things first-blown 240 did you mean 342's?

Nope, its a 2.43. It came from a 79 Camaro.

[70chevrolet]

i am sorry, I meant 27" diameter and not 27" radius..
I was planning on bumping my tire size to 28 inch but that doesnt help much.
That 2.43 ratio is looking even sweeter than 3.08

[Dino Driver]

I would talk to 2 diferent places, a tranny shop and a shop that specializes in rear ends. Take the gear ratios of your tranny and size of tires your running. Tell them what type of driving you do, IE: city driving, freeway to lets say 70MPH, what RPMs you expect to run at that speed or if you want a combo of city and highway, do you want to be fast off the line or quick acceleration once you're already moving. I have a 4L80 tranny and I plan to run 28" tires and I'm going to put in a 3.73 rear end.

[70chevrolet]

Originally Posted by Dino Driver

I would talk to 2 diferent places, a tranny shop and a shop that specializes in rear ends. Take the gear ratios of your tranny and size of tires your running. Tell them what type of driving you do, IE: city driving, freeway to lets say 70MPH, what RPMs you expect to run at that speed or if you want a combo of city and highway, do you want to be fast off the line or quick acceleration once you're already moving. I have a 4L80 tranny and I plan to run 28" tires and I'm going to put in a 3.73 rear end.

yeah but your 4l80e is overdrive. you can run those short gears..
your 0.75 final drive with 3.73 at 65 mph will give you about 2200 rpm
my 1.0 final drive with 3.73 at the same speed would be 3000 rpm
I figure that I need to be at about 2500 rpm or less doing 65.
which leaves me at 3.08 rear at 65 mph with a 1:1 final drive = about 2500 rpm..

[awbrown]

I dont think you will like the 2.43 very much unless you really only want gas milage , and then if you have any cam at all in the engine i dont see it getting very good milage unless it is real small...

[pmkrew]

5 speed OD trans swap anyone?

[70chevrolet]

I think I am going to go with 3.08
That is what factory gearing would have been on early 70's mild performance cars with muncie m20 and mild v8 performance. Usually the 6cyls or autos got the 2.73..
I need something I can drive.. So I won't be able to chirp 3rd gear anymore, big deal. Ripping burnouts around town gets old pretty fast.
I priced out tbd whole thing and for a 3 series eaton posi, Richmond 3.08 gears & a Richmond master install kit, i'm looking a $770 to my door from summit. I assume I will end up paying another $400 for the install from a quality reputable shop.
I could do a tremac 5 speed, but I'd be looking at double the cost. Not to mention I would still have a 50 year old, open rear diff out back. This way the drivetrain will be all new from front to back. And I get the eaton posi that I've wanted for some time.
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[blown240]

Originally Posted by awbrown

I dont think you will like the 2.43 very much unless you really only want gas milage , and then if you have any cam at all in the engine i dont see it getting very good milage unless it is real small...

This is a good point. My 51 gets 25 mpg on the highway at 70mph, and about 21 around town. But you need to keep the cams pretty mild.