The Stovebolt Site Forums The Shop Area Driveline Rear Differential

Hi! I have a stock 1961 Apache C10, problem is i would like to be able to get up to highway speeds with it. Im thinking about swapping out teh rear dif, mainly just the ringa nd pinion. This is not the only thing i am doing, i will EVENTUALLY, (money permitting), slip a new fourth gear on their. I have a straight 6 235 with a four on the floor. Im wondering what bolt rear dif i could swap in? (8, 10, or 12?) and what size ring and pinion? I aint drag racing, just want to be able to reach about 65 without doggin it and wearing out my engine Got any helpful suggestions? Also what years could i use? because i will probably be looking in the junkyard also. And what should it run me in $?

12 bolt truck rearends fit pretty well.

My opinion is it is much EZer to swap transmissions to get better top end speed than it is to swap rearends.

Look into overdrive transmissions such as the t5, and a833. There was even an OD transmission that was optional for your truck that can be picked up fairly inexpensively. You will likely have to have a new driveshaft made.

The stock gears work very well with OD transmissions.

John

I agree with John, an overdrive transmission is the way to go.

is an OD tranny still the best way to go if i want to keep all the tourque though? Wont that re-gear it for less tourque?

the OD trans doesn't change the lower gearing, it just adds a new "high gear" of about 0.7:1, which you cannot get for your original 4 speed ["I will EVENTUALLY, (money permitting), slip a new fourth gear on"] .... no effect on "torque", just gives you higher top speed at the same RPM

Bill

I suppose it would depend on a few things. Compared to the original 4 speed, most overdrive transmissions have a much higher gear ratio in 1st and 2nd gear, which can affect the final drive torque for getting the load moving. However, the overdives do tend to have better gear spacing, making for easier driving with less rpm drop between gears.
With an overdrive, you would still have a 1:1 final drive gear option, same as the stock 4 speed. That would make no change. Once you're rolling and up to speed, shift into overdrive and you get the lower rpm you're looking for. Changng the final drive gear in the differential would actually cause the final torque output to be reduced in all gears, not just the final gear in the transmission. Using an overdrive is really the best of both worlds, you still have the diff gear for touque, but the advantage of a highway gear for economy and reduced engine wear.

I wasn't going to mention the NV4500 as an option...

As LB55 said... getting a freeway friendly differential will reduce available torque and acceleration in every gear.

Assuming you have a sm420 in your truck now, first gear is not used much. However when it is used there is a reduction of about 27.5:1... meaning your engine will spin almost 28 revolutions before your tires will spin 1 revolution. This reduction maxismizes and multiplies the torque.

The sm420 has about a 7.05:1 first gear, and no overdrive. An NV4500 has a pretty low first gear (5.61:1) and a 0.73:1 od. The gear spacing of the nv4500 from 1-4 is almost identicle to the sm420 in 1-4. The big payoff is the OD gear. There is a slight loss of torque in 1st gear, but a big drop in rpm in 5th.

So to answer your quetion. Changing the differential will affect acceleration and applied torque in every gear. This could make your 235 feel sluggish. Swapping the granny gear for the OD gear will give you more speed, but less takeoff power at the boat ramp. Using a NV4500 (likely overkill on a 235 powered 1/2 ton) will give you a bullet proof transmission that can pull a boat out of the water and down the road reasonably well.

Every modification you make to the truck over the original confiuration is a trade off. If you use an OD transmission, you lose the granny gear. If you add add an OD behind the transmission, you can keep your granny gear, but you will lose your lunch money for a year. The NV4500 swap is a rather expensive swap. The t5 swap is a fairly inexpensive swap.

As far as Stand alone Overdrives, there are a few available. One is a "Ranger Overdrive". This unit goes between the bellhousing and the transmission. It will push your existing shifter lever back about 6.5 inches, and an an extra shifter for the ranger.

Another Stand alone OD is a Gear venders OD. These typically cost about $2700 new, and go between the transmission and the rear axle. This unit as well as the ranger unit allow you to gear split giving you about 8 possible forward gear ratios.

What is the C10's rear end ratio? I go 65 easily in my '58 with 3.90 rear (and front, though not being used on the road) gear ratio. 32" tall tires help.