Gear Ratio Swap

[03BlkZ]

I have a 70 C-10 short bed that I assume has a dana 60 12 bolt. It was factory with 3.73 gearing and is also a 3 OTT truck. I am running 255/70 tires on my 15 inch rally wheels. Now that my restoration is complete I'm thinking I want to change the gear ration to maybe a 3.42 to help lower the RPMs the engine it turning on the highway. This is by no means a hot rod truck and it does fine around town and up to about 60 mph but the engine sounds like it's spinning pretty good. I don't have a tach so I'm really not sure. The question is how hard is it to swap the gearing to something like a 3.42. There are a lot of car shows I want to go to that are within driving distance but with that gearing I really don't like to get over 55. Some of these would require interstate highway driving and I don't feel safe doing that at 55 to 60. I prefer to drive my truck over trailering.

[72SB]

I am running same size tire and rear gears with 3 OTT. Swapping to 3:42 will reduce RPM's at X speed but not by alot. 3:08 would be a bigger change and if you don't pull a trailer or haul heavy loads (what truck was principally designed for) the 3:08 ratio would be a more Hwy friendly gear

There are several online RPM calculators you can use where you enter your tire size (diameter), gear ratio & tranny 411 to show what RPM at X mph you will get

You can find tire diameter by searching your tire brand and size and in specifications it shows tire diameter

Also if your current tires are bigger than stock your speedo is off unless you have changed the driven gear in trans. When I went to 255/70x15 I went with 1 tooth less and its still about 3-4 mph off (reads slower than actual) so your "real speed" now at 55 may be closer to 62

Phone APP like SPEEDBOX is handy to see actual speed (GPS)

online RPM calculator

https://spicerparts.com/calculators/...rpm-calculator

[cj847]

A gear swap is not a good do it yourself project. In my opinion, the $300 labor is a bargain.
It is an expensive proposition:
Gears- $300
Bearings - $200
Carrier - $75
Labor - $300

PS> the rear is a GM truck 12 bolt, not a Dana 60, and not a GM car 12 bolt.
If it were me, I'd put in an overdrive transmission if your current rear end isn't making funny noises.

[kwmech]

Your drop to 3.42 will only yield you 9 percent difference. What engine is pushing the truck down the road?

[03BlkZ]

Originally Posted by kwmech

Your drop to 3.42 will only yield you 9 percent difference. What engine is pushing the truck down the road?

The truck has the original but rebuilt 350.

[kwmech]

going to a 3.08 will be your best bet if you do not plan on doing any towing. If it is just a show and enjoy truck you'll be fine. that is unless you want to change out the trans to a 5 speed

[71meangreenc10]

If you are keeping the 3OTT, go 3.08. That is in my SWB and it is a good set up. Mine started as a 6 Cyl and 3.73. I went 2.76 on the first gear swap and the truck fell flat on its face with the manual trans. Then went 3.08, truck liked it. I run 275-60-15's on it.

Also, (things may have changed) you may have to change the carrier to go to 3.42 from a 3.73 carrier. At least that is what Randy's told me many moons ago.

Smitty

[jayoldschool]

Keep the 3OTT, keep the gears, and add the factory OD on the back of the trans!

[Accelo]

I agree with the excellent advice given by the board members. I just want to add some information.

Rpm difference between 3.70 and 3.42 is only 188 rpm at 60mph.
I agree that moving to a 3.08 gears makes sense for you application.
It would drop your Rpm by 430.

12 bolt truck applications have the following carrier breaks: 2.73 & Numerically Lower, 2.76 - 3.42, and 3.73 & Numerically Higher. So a carrier change is required if using factory parts/options.

When you purchase gears some manufactures make a thicker gear set so you don't have purchase a new carrier to make the change. YOU can only go one direction or the gear gets to thin. Not an option moving to 3.08's from a 3.73 carrier.

I personally like the factory OD option mentioned by jayoldschool. Especially since you want to keep the 3 on the tree. Add the Overdrive to your transmission. The OD units are not that difficult to find. The wiring and switches are more difficult. They work sweet, floor it and they automatically shift out of OD and back up when you let up on the gas. The down side is you have to shift out of OD to have engine braking going down hill.

I will tell you why we like our OD's. If you installed the typically OD automatic your rpm, at 60mph, would drop to 1800rpm. A 30% drop.
You would have to change the differential to 2.59 gears to get the same RPM drop. It becomes a no brainier if you were an engineer. I understand wanting to stay with the TSOTT. (three speed on the tree)

Cheers.

[LNP]

Check out Gear Venders first to see if they can set you up a overdrive. It bolts to the trans.

https://www.gearvendors.com/index.html

[tutone]

In my earlier post, I told you the perils I had with my swap to a 3:08 from a 3:73 back in the early 80s. I did some research and the 3:08 would have been ok with a proper saginaw. Mine was a used truck and it was entirely possible that the tranny could have been replaced with a 3 speed with a higher,{numerically lower} first gear. If you have a pretty good idea that your trans has a lower,{numerically higher} 1st gear, then the 3:08 set will come much more likely to work. I have had good success with 3:08s behind m20 muncie 4 speeds in lightweight camaros. You will find the perfect recipe.

[03BlkZ]

Originally Posted by Accelo

I agree with the excellent advice given by the board members. I just want to add some information.

Rpm difference between 3.70 and 3.42 is only 188 rpm at 60mph.
I agree that moving to a 3.08 gears makes sense for you application.
It would drop your Rpm by 430.

12 bolt truck applications have the following carrier breaks: 2.73 & Numerically Lower, 2.76 - 3.42, and 3.73 & Numerically Higher. So a carrier change is required if using factory parts/options.

When you purchase gears some manufactures make a thicker gear set so you don't have purchase a new carrier to make the change. YOU can only go one direction or the gear gets to thin. Not an option moving to 3.08's from a 3.73 carrier.

I personally like the factory OD option mentioned by jayoldschool. Especially since you want to keep the 3 on the tree. Add the Overdrive to your transmission. The OD units are not that difficult to find. The wiring and switches are more difficult. They work sweet, floor it and they automatically shift out of OD and back up when you let up on the gas. The down side is you have to shift out of OD to have engine braking going down hill.

I will tell you why we like our OD's. If you installed the typically OD automatic your rpm, at 60mph, would drop to 1800rpm. A 30% drop.
You would have to change the differential to 2.59 gears to get the same RPM drop. It becomes a no brainier if you were an engineer. I understand wanting to stay with the TSOTT. (three speed on the tree)

Cheers.

Awesome! Thanks for the advise. I will start researching the OD option.

[cwcarpenter98]

So I have the Borg Warner 3 speed overdrive setup in my 63. Swapped it in a couple years ago. Been driving around with it a bunch since then.

Before I swapped in the overdrive, I was spinning my 283 about 3k rpm to run 70 with the same 255/70 15s and 3.73s. With the overdrive trans, my rpm dropped to around 2400 rpm to go 70. I went from around 12 mpg on the interstate (no overdrive, carb was a little rich at idle, and no fan clutch) to my most recent interstate mpg of 16 (with overdrive, carb tuned up a little better, and a fan clutch). Those are really guesstimation numbers, but still was well worth the swap.

You have two options though. You can get one that's the 4 bolt side cover Muncie version or 7 bolt side cover Saginaw version. Try to match up with whatever current side bolt cover you have so that your shift linkage won't need to be modified. You will want to get a whole overdrive transmission because the overdrive transmissions have a lockout rod that is automatically engaged when shifting into reverse so that you don't wreck the sun and planet overdrive gear.

Originally Posted by Accelo

The down side is you have to shift out of OD to have engine braking going down hill.

I disagree with this statement. The overdrive does still engine brake when in overdrive if you are above the 28 ish mph cut in speed determined by the governor. Engine braking is reduced a smidge because of the overdrive, but it still works. Once you drop below the cut in speed and the trans automatically shifts out of overdrive, then you will lose engine braking because the overdrive unit freewheels under that speed. It's not bad though and definitely something you learn to control fairly quickly.

[Accelo]

cwcarpenter98 thanks for the correction.
Been a long time since I drove one. I purchased a 1966 Chevelle with one installed this was likely in the 80's. It didn't work when I got it but I managed to fix it by cleaning up the governor. The OD's come with a switch on the carburetor linkage that manages the kick down out of overdrive. Really neat system.

Here is a link to one on Fleabay;
https://www.ebay.com/itm/14462712109...UAAOSwDddivN6N

The large can on the side is the governor that controls the minimum engagement speed cwcarpenter98 spoke of.
It's the controls that are difficult to find and get operational.
Should also be a cable that is inside the cab that is operator engaged.

The EBay price is $699.99 and $150 shipping which comes to $850.00, seems expensive. So are new carriers and bearings and gears.
But I loved driving these OD's and not days they would be quite the novelty.


Cheers.

[72SB]

OP's tire size, 255/70 R15, is a 29.1" diameter tire

with 3:73 gears at 65 mph the RPM is 2,799 RPM. (3 speed manual trans)

IMHO 2800 RPM is no big deal for sustained driving. Yeah a little more gas but the engine can run at that RPM all day

at 70 mph the RPM is 3,017