Rearend questions

[Kiwi]

I have a 1971 C10 (V-8, automatic, 2WD, long bed, 5-lug wheels). I recently installed a new TH700-R4 overdrive transmission and now the speedometer is reading too fast. I'm trying to determine what speedo drive gear I need to buy/install in the TH700-R4 ... which will be based on two factors:

- Rear tire size (I know this answer)
- Rear gear ratio (I don't know this answer)

1. What were the factory rear gear ratios available in 1971 for a C10?
2. Is the rearend ratio listed somewhere on a trim tag?
3. Did some/all of the C10 rearends come with a limited-slip differential? If so, does the limited-slip require using the liquid posi additive (such as with 1960's 10- and 12-bolt rearends) when changing the rear diff oil?
4. What type of rearend came in a 1971 C10 (I think it was a "Dana" ... but a Dana what??)?

[Jacfourteen]

Prob a 12-bolt rear. as far as the lim. slip and the gear you can look to see if the metal tags are still attached to the cover bolts, but they are prob long gone. The easiest way is to pull the cover and see for yourself. Post up the #'s on the ring gear and we can figure it out. Fresh fluid never hurts anyways.

[Kiwi]

I know about counting the teeth on the ring and pinion gears ... but I was hoping to be able to learn the ratio without having to pull the rear diff cover.

[69nova]

If you put the rear of the truck on jack stands and turn a wheel and the wheels turn in the same direction then you have limited slip and you will need the additive. They turn in opposite direction if open differential. Also if you turn the wheel one complete rotation and count the number of times the driveshaft rear yoke turns then that will be your ratio. (3 3/4 turns = 3.73 etc.) There is a catch to this if you have a non limited slip rear though. You need to have one tire stationary and the count will be half so rotate tire two rotations.

It can be a pain to pull the rear cover and refill with oil if you dont have too!

[Jacfourteen]

Originally Posted by 69nova

If you put the rear of the truck on jack stands and turn a wheel and the wheels turn in the same direction then you have limited slip and you will need the additive. They turn in opposite direction if open differential. Also if you turn the wheel one complete rotation and count the number of times the driveshaft rear yoke turns then that will be your ratio. (3 1/4 turns = 3.73 etc.) There is a catch to this if you have a non limited slip rear though. You need to have one tire stationary and the count will be double. (6 1/2 divided by 2 = 3 1/4 = 3.73)

It can be a pain to pull the rear cover and refill with oil if you dont have too!

This is a good way to get an idea for the ratio. One small note though, it would be 3 3/4 turns equals 3.73, not 3 1/4.

[69nova]

Thanks. I caught my mistake after I posted it. 3 3/4 = 3.73 (roughly) Sorry for the brain fart!

[Wrenchbender Ret]

That trans uses a large driven gear for the speedo & is easy to change & calibrate. Match the odometer reading against the mile markers on the freeway. Like your odometer reads 2.4 in between 2 mile markers. Take the gear behind the speedo cable out of the trans. & find or count the number of teeth. It is cast in the edge of the gear. Being yours is fast you will want a gear with more teeth. They are available from parts stores in the Motor-mite line or from a trans shop or dealer. The holder will acept several sizes & they are marked on the houseing. You can get away with one size + or -. If it is far off you will need a different houseing for the gear.

[raycow]

If you are counting the gear ratio on an open rear, make sure one wheel is on the ground or otherwise held so it can't turn. Turn the free wheel exactly TWO revolutions. The number of driveshaft revolutions will then be equal to your axle ratio.

Ray