Pretty sure what I say below is accurate.
All factory 3 speeds and 4 speeds have a final gear ratio of 1:1.
The exception would be a factory 3 speed with the solenoid style OD gear, and I’m not sure those were available in these trucks from the factory.
All TH350 and Powerglide transmissions have a 1:1 final gear.
The 700R4 transmission has an OD gear.
Most of the 1960-72 C10 tucks had a 3.90, 3.73 or a 3.42 rear axle. I’m sure some were 4.10 too.
An engine at any given RPM will turn the wheels faster or slower based on the transmission gear X the rear axle gear.
So if your engine has higher RPMs than you desire at say 65mph, and you have a transmission with a 1:1 final drive gear, then you can bring the RPMs lower by either changing to a different rear axle gear (numerically lower like 3.08) or you can change the transmission to add an OD gear. The trick is to find a happy place where the truck has power to get going in 1st gear and yet not lug the engine in the final gear.
Example: Some folks with a 3.73 rear axle change to a 3.08 axle. They say it drives really well on the freeway with the factory 3 on the tree and a transmission final gear of 1:1. The RPM calculator I use tells me that combination will get the RPMs at 2320 while cruising at 65MPH (Assuming a 29” tall rear tire.). However, they have effectively reduced their power in 1st gear for things like pulling trailers and heavy loads. They have effectively “lost power” in all gears. That’s ok depending on how you use the truck.
Some folks install an OD transmission to reduce the highway RPMs and leave the rear axle as is. An OD gear can be a good solution, but not always. The engine must have enough power to move the vehicle when in overdrive gear or the driver will be constantly downshifting at the slightest upgrade.
Modern drivetrains usually target RPMs in the 2200 range on the highway. I owned a Miata that had a small short stroke engine that had RPMs above 3000 on the highway. That’s because that little Mazda engine didn’t make much power in the lower RPM range being that it was a short stroke engine.
So if you keep the transmission as is, you can reduce your highway RPMs with a numerically lower rear axle gear, but at the cost of losing some power in all gears.
If you install a transmission with OD, a 3.55 gear X 0.86 overdrive will get you 2299 RPMs at 65MPH with a 29” tall rear tire.
Finally, simply changing to a taller rear tire will functionally reduce the RPMs too. It all depends on what stance and look you want.
