Stock Rear Axle Gearing

[Chulisohombre]

It will definitely hurt the power a little bit with a gear change. These old trucks were built when average speed limits were 55 mph. I also don’t take mine over 3k rpm for long periods either. Which means a top speed of 60-65. I just live with it because I know my trucks limits and don’t generally drive it more than 50-100 miles daily trips. It was my daily driver for a long time though. Highway driving was kinda sucky getting passed all the time but without od or better highway gears you are kinda stuck with what you have. A axle swap is almost always cheaper than regearing a axle if you can find a good complete axle in the ratio you want. I went 4.10 when I changed to 3/4 ton axles because I’m also running 35 inch tires. But when running stock tires somewhere in the 3.42-3.73 would have been best. 307 isn’t the strongest of engines either so the lower gearing helps keep you in a good power range for pulling. It’s a toss up between power and speed that is hard to compare unless you try it.
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[King951]

So I installed a tachometer on my truck. 12V + tach wire off my HEI distributor. I also used a GPS speedometer to get a semi accurate reading of how fast I am going. At 60 MPH I was at 3,000 RPM. I used an online RPM calculator taking my tire size into account and it read my rear gear would be around 4.6 give or take a little. So I am pretty certain I have a 4.56 rear end. I do enjoy the pulling benefit of the 4.56 but going 55 MPH on the freeway gets old pretty quick. An overdrive sounds like a nice alternative. Using the same calculator and the fact that my new tires will be about 1 inch taller, if I had a standard overdrive I could cruise at 70 MPH around 2550 RPM which sounds much better. Only issue is an overdrive transmission conversion sounds like a big job. I found a local shop that sells rebuilt 700R4 transmissions for about $450 with a 2 year unlimited mileage warranty. Maybe one day. Thanks for all the feedback so far everyone.

[Chulisohombre]

The bigger tires will help a little but probably only a couple hundred rpm. I’d love to throw a nv4500 in mine to have a overdrive someday. That’s not a bad price for a rebuilt 700r4. More than likely they will want you to either pay a core charge or bring in a good core for rebuild at that price.
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[King951]

I am a hardwood / laminate contractor and usually travel anywhere from 5 - 60 miles to am from a customer's house. The amount of freeway driving I do on a day to day bases usually isn't anything extreme, but at the same time 55 MPH with my engine over 2500 RPM starts to drive me crazy, especially days like today. An overdrive transmission swap would probably be the most ideal solution, but I need to be realistic as I can't really afford such a large amount of downtime with my truck anytime soon, and after looking up other members 3 speed manual to auto swaps, the cost can get pretty high once you throw in all the little bits needed to actually get going. I am using an online calculator that takes into account final drive gear, RPM, tire size, rear end gear, to determine what RPM / speed you can cruise at. My final gear is a 1:1, my rear end in a 4.58, my new tires are 30.5 inches tall. This comes out to 59.72 MPH @ 3,000 RPM which sounds about rite. Even the semi trucks in the slow lane pass me up! I would like to keep my RPM closer to the 2,500 range and my speed in the 60+ MPH bracket instead of 50 MPH at 2,500 RPM with the 4.58.

No overdrive (1:1 final gear)
4.58 @ 2,500 RPM = 49.55 MPH
4.10 @ 2,500 RPM = 55.35 MPH

Still not great, but better. Going from a 4.58 to a 4.10 doesn't seem like it is going to make the word of difference, but 6 MPH increase isn't bad when you're only going 50 to begin with.

Overdrive 0.7 final gear
4.58 @ 2,500 RPM = 70.78 MPH

This seems like it would be the best option as I would also get to keep my high rear gear to help with moving at the lower RPMs. I also probably wouldn't travel around at 70, probably closer to 60-65 MPH. 65 MPH would be 2,300 RPM which sounds like a dream come true.

Does anyone know of any manual overdrive transmissions that bolt up to a SBC 3 speed column bolt pattern? Or in other words an overdrive manual transmission that wouldn't be too hard of a swap? I feel like that may be my best realistic option as in order to get optimal RPM / speed by changing my rear end I would have to lose too much pulling power to make it worth while. Overdrive just seems to be like the perfect in between.

[Chulisohombre]

There’s no perfect direct trans swap unfortunately. A nv4500 is a close swap that’s been done before swap for you. Five speed and some small modification is to put a hole in the transmission cover for the floor shifter and figure out how to hook up a hydraulic clutch. I’m not sure if lengths of the transmission is very far off so you would maybe have to modify the crossmemember and driveshaft as well. A rear end swap to 3.73 would give you close to the same change in final gearing and be a cheaper solution. But of course pulling power would be affected. But you could run closer to 65 at a lower rpm.
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[King951]

Update with some photos.

My rear end is in fact a Dana 60. I took a flat head screwdriver and cleared some of the caked on grease so I could read the tags / stamps on the rear end itself. I put the truck in neutral on a flat surface and moved the driveshaft by hand. There was about an 1-2 inches of play in the driveshaft but it didn't appear to be from any of the u-joints, it appeared to have the play in the rear end itself. I also noticed that for example if I have the truck in 3rd gear and let off the gas and then step back on it the truck makes a clanking sounds once it picks back up. I assumed it was a u-joint at first but am beginning to believe it has something to do with the rear end. Other than that I have had no issues driving the truck 500+ miles this last week. I did put some new gear oil in the rear end when I first got the truck.

I don't really know what steps I should take at this point. I would like to change the rear gear to something like a 4.10 instead of a 4.56, and it appears as though there may be an issue with this rear end in the first place. Not too sure where to begin as I have never done work on a rear end before and I would like to minimize the truck's downtime as much as possible as I am still using it for work and my daily driver almost every single day.

Any information is much appreciated.

Tags state the following
60
C-30255
41-9
4 56


https://imgur.com/a/LxsZw

[Bigdav160]

There was about an 1-2 inches of play in the driveshaft

that slack at the pinion gear sounds pretty normal

[King951]

Originally Posted by Bigdav160

that slack at the pinion gear sounds pretty normal

Normal as in nothing I should be worried about?

[Ol Blue K20]

Definitely a 4.56 according to the tag.

[Mike C]

There should be very little rotational movement at the pinion. There will be more if your truck has the Maximum Traction differential however.

Overdrive IS the answer if you are going to drive the truck a lot!

[Hotcorner]

FWIW, 3.73 gears and 33" tires = 2800 RPMs @ 65 mph, but that's an SM465 trans. Speed verified pretty spot on by GPS.

[Chulisohombre]

The gears shouldnt have that much free play in them before turning the axle. Maybe half inch or so. The clang you here sounds like the ring and pinion reingaging and slamming together from slop. Only way to know is to get inside it and look at how much backlash the ring gear has to the pinion. Your spider gears could be getting worn as well. Witch would take more rotation to start moving the axleshafts as well. If you have a locker there may be a little more slip and the ping you here is the ratcheting if the gears as you give it gas. That usually doesn’t happen except around corners though. That tag definitely shows 4.56 gearing though. I’d pull your diff cover and inspect the gears for wear though. It’s good to take a look and replace the fluid every so often either way.
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