3.73 gear swap for ????

[lakeroadster]

I have 3.73 gears in my stock C10 12 bolt differential. Due to having a 4 speed muncie and 33" tall tires my clutch and my attitude would benefit from a higher gear ratio set.

What is available as a direct swap without having to change a lot of pieces parts?

Thanks in advance.

[LILRED66]

A 3.08 posi or single-trac, as well as, as 3.56 posi or single-trac were available options in the 4-5-6 Chevy C10 trucks. They would be a direct swap and retain the six lug hubs, otherwise about any late sixties, early seventies rearend would suffice, but may involve some minor modifications to the shock mounts, Panhard bar mount and trailing arm pads.

[lakeroadster]

The 33" diameter tires and the 2.20 Muncie 4 speed 1st gear means more ratio is needed, not less.

So if possible I am really just wanting to change the center section out for a higher ratio, like a 4:11 or ???

Anybody know whats available ratio wise?

Thanks

[Captainfab]

I believe that 4.10 was the lowest gear available from the factory. The 12 bolt was in use thru '82 in the C10 and K10 trucks. They are more common thru '79. However if you are willing to go with aftermarket gears, Richmond has them all the way to 5.38's and Yukon goes down to 4.88's. Those will fit the 4 series carrier you currently have.

[LILRED66]

4.11:1 single-track and 4.11:1 positraction rear axles were available, per the 1966 CHEVROLET Truck Data Book (black dealer-level 3-ring binder).

[JRBECK64]

I took a 4:11 out of my Sons 64 and put a 3:08 in it because the 4:11 was rattling my teeth out of my head at 65 MPH on the freeway. The tires that I had on it when I bought the truck were only 26 inches tall, I replaced them with 30 inch tall tires and with the 3:08 gear in the back its very comfortable at 70 now. I cant even imagine what the top end is on this thing now. Even with the 33 inch tall tires It might be worth the time to find a RPM calculator to figure out what you will have when all is said and done. If you google RPM calculator you will find that you have to put in Tire Size and Gear Ratio and speed to get the RPM's. Might be worth your time.
JRBECK64

[TwoFiftyShifter]

My vote goes for a 4.56, especially with those meats. I drive pops Camaro with an M22 and it has 4.11s now, it was better when it had 4.56. Nothing out on top for for street driving it had great punch.

[Winola73]

To calculate RPM at a given speed the formula is:

Gear ratio X Transmission ratio X 336 X MPH / Effective tire diameter = RPM

"Effective" tire dia. is the true rolling diameter under a load. A 33" tire is not really 33" tall when loaded and it can make quite a difference in your RPM.

Here's a website with some very useful calcualtors....

http://tri-5.chevyrides.com/speedo.php

[lakeroadster]

I am with ya guys and I have crunched all the numbers.

The one thing many overlook is Starting Line Ratio, aka as Launch Ratio.

Starting Line Ratio = first gear ratio * rear end gear ratio, 9-11 is ideal. However large rear tires hurt this number.

My current configuration, 2.2 first gear and a 3.73 ratio rear yields a starting line ratio of 8 but the 33 inch tall tire hurts this number. As originally stated my clutch would benefit from a higher gear ratio set and the truck fun factor for non-interstate driving would be greatly multiplied.

That is why I am thinking a 4:11 or higher would be a better choice and a 4:88 would be a ton of fun!

Numbers with a 3.73 gear @ 2600 RPM
1st 31 mph
2nd 42 mph
3rd 53 mph
4th 68 mph
Starting line Ratio = 8.2

Numbers with a 4.11 gear @ 2600 RPM
1st 28 mph
2nd 38 mph
3rd 49 mph
4th 62 mph
Starting line Ratio = 9

Numbers with a 4.88 gear @ 2600 RPM
1st 24 mph
2nd 32 mph
3rd 41 mph
4th 52 mph
Starting line Ratio = 10.7

[msgross]

3.73 was the largest factory option wasn't it?...

they make gears higher than 3.73 for the three series carrier as stated, www.ringpinion.com has plenty of options... I would think that 4.11 would be plenty, but I don't know what your setup is...

http://www.ringpinion.com/DiffDetail...%26DiffID%3d34

[Rich 5150 69]

Then mine should look fairly good when done, 400 sm blk, 7004r, with 373, 28 tall tire looking at 10.7 ratio....

[Heater63]

I am not an expert, but just went through this, and as msgross mentioned, 3:73 is the largest you can go on a "3 series" carrier. To go 4:11, et al, you have to change to a 4 series posi.

I just had a Detroit TruTrac posi put in mine, and stayed with 3:73, but I will be running 5 speed overdrive TKO 500 with 28 inch tire. 70 mph = 2200 rpm

Have you thought of going with overdrive? best of both worlds, you can have tall gearing and decent cruise rpm.

[lakeroadster]

Originally Posted by Captainfab

I believe that 4.10 was the lowest gear available from the factory. The 12 bolt was in use thru '82 in the C10 and K10 trucks. They are more common thru '79. However if you are willing to go with aftermarket gears, Richmond has them all the way to 5.38's and Yukon goes down to 4.88's. Those will fit the 4 series carrier you currently have.

Ok, more questions on this dated thread. So if i go with Richmond Gears I can use a 4.56 gear. Next question, has anyone here changed their own gears? Is this something that can be done at home or does it require specialized tools and/or big capacity impact wrenches?

Also, if I currently have 3.73 gears that means I have a 4 series carrier, right?
My information states that for stock trucks using the GM 8.875" 12 Bolt Differential:

3-Series: (3.42 and numerically lower)
4-Series: (3.73 and numerically higher)

Thanks...

[markeb01]

My only experience with changing gears has been changing the pumpkin on a 9” Ford, and the Dana 44 in my truck. In the Dana, 3.70 is the carrier break. 3.70 and down (numerically) like 3.08, 3.23 uses the same carrier, and anything with a lower gear ratio (higher numerically) like 3.90 or 4.10 requires the 4 series carrier.

Setting rear end gears is one of the rare tasks I don’t do myself. From what I’ve read it requires a dial indicator, and a lot of patience (experience helps), but the few times I’ve changed gears I had so much money invested in parts I wasn’t willing to risk it with a bad setup.

One other note – a while back I had a link to a website that compared all the different brands of rear end gears. It listed them by brand, ranked them by quality and if they made the gears or simply repackaged them. My hard drive crashed and I lost all that, but I remember their comments about Richmond gears – claiming they were harder to set up than other brands, ran hotter and were noisier in use than other brands. This stuck in my head because I just bought a Richmond 4 speed and was shopping for rear end gears. Not sure if any of that matters in a hot rod, and it certainly doesn’t if they’re the only brand that offers the ratio you want.

[Captainfab]

You are correct on the ratio breaks for the series of carriers. As for setting up gears, many guys won't attempt it. Both my son and I have done a few sets with success. You will need a dial indicator with a magnetic base, a press for removing and installing bearings, and a lot of patience. I would suggest reading up on the procedure or investing in one of the instructional DVD that are available. The only thing you could need a large impact or breaker bar for is for the pinion bearing cruch sleeve, but there is an alternative to that. If you decide to have a go at it, let me know if you have any questions.

Originally Posted by lakeroadster

Ok, more questions on this dated thread. So if i go with Richmond Gears I can use a 4.56 gear. Next question, has anyone here changed their own gears? Is this something that can be done at home or does it require specialized tools and/or big capacity impact wrenches?

Also, if I currently have 3.73 gears that means I have a 4 series carrier, right?
My information states that for stock trucks using the GM 8.875" 12 Bolt Differential:

3-Series: (3.42 and numerically lower)
4-Series: (3.73 and numerically higher)

Thanks...

[lakeroadster]

Thanks guys!

[blackedoutharley]

My question is: "other than the possible abuse to the clutch when taking off from a standing start are you otherwise happy with the performance of the truck with the current gear/tire set up?"

If so, have you considered going to a heavier flywheel (or more accurately; what flywheel do you currently have?)?

p.s. I can't say enough how much I like your truck and specifically the fact that you put a .... a truck tire on .......... (GASP!) a truck!

[TwoFiftyShifter]

Must have good tracton for a truck with those manly meats out back, be a killer combo with a 4.56 or deeper gear

[markeb01]

A bit more food for thought on the rear axle ratio – the street tires on the back of my truck are 29.1” tall. The truck normally accelerates nicely with a launch ratio of 10.8 (2.64 1st gear x 4.09 rear axle).

I recently swapped over to my snow tires which are 27.4” tall and the truck accelerates like a bullet. I wanted to determine how much of a difference the tire diameter made in final drive. Using a standard gear ratio calculator here’s what I discovered:

To achieve 2600 RPM with the 27.4” tires (actually 2608 rpm) and 4.09 axle the truck is traveling 52 mph.

To achieve the same 2600 RPM with the 29.1” tires requires a 4.33 ratio, and the relative launch ratio changes to 11.4. I actually enjoy the harder acceleration with the shorter tires, but the engine runs out really fast.

With a 2.20 low gear, a 4.88 rear axle would provide nearly the same launch ratio I have at 10.7. And a 4.88 with a 33” tire will also turn 2584 rpm at 52 mph. It won’t make a very efficient freeway cruiser, but would certainly be fun around town.

[Greshampiglet]

My 64 GMC came with 4.11 factory I went with 3.54's I still have the 4.11 here in Oregon

[lakeroadster]

Originally Posted by blackedoutharley

My question is: "other than the possible abuse to the clutch when taking off from a standing start are you otherwise happy with the performance of the truck with the current gear/tire set up?"

If so, have you considered going to a heavier flywheel (or more accurately; what flywheel do you currently have?)?

p.s. I can't say enough how much I like your truck and specifically the fact that you put a .... a truck tire on .......... (GASP!) a truck!

Yeah, a truck tire on a truck, I guess I am just insane...

Never even thought about the heavier flywheel idea. I agree that for drag racing it would help launching the the truck off the line, but then again I don't know that a heavier flywheel would help to minimize clutch wear on a daily driver. And I am thinking that the heavier flywheel will also hurt mpg when driving around town too. But I have no experience of comparison, heavier vs. stock.

I think it has a stock flywheel at this point. Any first hand advise in regards to a downside to the heavier flywheel?

As for the current performance, when I bought the truck it had 31x10.5-15's on the rear. I was happy with the performance. Now that I have the 33x12.5-15's I have not driven it except in our driveway 'cause I have the bed off of it and due to grafting in a '67 Camaro dash the gauges are not hooked up yet.

I am holding off on the gear change until I get the truck back on the highway, next spring ? But I can tell from driving it in our driveway that the clutch needs to be slipped a bit than before to get it moving.

[blackedoutharley]

The only "downside" to the heavier flywheel in a truck application is trying to hold it in place while you start the bolts as you lay under the truck, LOL.

My truck has a 45lb flywheel and you can almost take off from idle. This is behind a healthy small block using a 12" clutch and the original SM420.

In drag racing we always wanted the lightest flywheel possible because it allowed the engine to rev freely. Because I launched at 4800 rpm there was no need to slip the clutch. This was using an 11" clutch and I simply sidestepped it.
In the dragster we used a 10" multidisc clutch. It was light to allow the engine to rev quickly.

For an example of the use of a heavy flywheel think Harley.... The heavy flywheel in a heavy vehicle converts the energy from inertia or centrifugal force to rotational energy which moves the car.

If you take the flywheel from a factory manual trans v-8 Nova and compare it to the flywheel from a manual trans v-8 pickup I would bet the truck flywheel is much heavier.

[BGM 69]

Should I be going from my stock gear ratio to a 3:73 limited slip or a 3:73 locker (automating locking diff)?

[Palf70Step]

What are you using your truck for is the main deciding factor, as well as what engine tranny combo are you using?

[luvbowties]

Originally Posted by Rich 5150 69

Then mine should look fairly good when done, 400 sm blk, 7004r, with 373, 28 tall tire looking at 10.7 ratio....

Other than engine, mine is identical to yours...and 1st gear is overwhelming/breathtaking. Used an aftermkt. 'vette governor & weight/spring kit(~$16.00, builder's cost), played with the adjustments to it, & got it shifting dead on 6000 rpm; and my 350 vortec just loves 1st And second--never let it climb all the way thru 3rd as no place to "legally" go that far.
Sam