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#1 |
Registered User
Join Date: Mar 2010
Location: Los Angeles, California
Posts: 81
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Rearend questions
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??)? |
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#2 |
Big Red - Now its a SWB!
Join Date: Jun 2009
Location: Oroville, Ca
Posts: 1,624
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Re: Rearend questions
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.
__________________
68 3/4 ton ![]() ![]() 41 3/4 ton ![]() 01 Tahoe LT 4WD (my daily driver)-Magnaflow Muffler, Rollin on 22's 02 Ford Mustang Convertable (Wife's Car)-Flowmaster 50, Clear corners, Intake, sittin on 18's |
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#3 |
Registered User
Join Date: Mar 2010
Location: Los Angeles, California
Posts: 81
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Re: Rearend questions
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.
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#4 |
Registered User
Join Date: Jan 2009
Location: virginia
Posts: 49
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Re: Rearend questions
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! Last edited by 69nova; 08-27-2010 at 01:57 PM. Reason: wrong info for ratio |
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#5 | |
Big Red - Now its a SWB!
Join Date: Jun 2009
Location: Oroville, Ca
Posts: 1,624
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Re: Rearend questions
Quote:
__________________
68 3/4 ton ![]() ![]() 41 3/4 ton ![]() 01 Tahoe LT 4WD (my daily driver)-Magnaflow Muffler, Rollin on 22's 02 Ford Mustang Convertable (Wife's Car)-Flowmaster 50, Clear corners, Intake, sittin on 18's |
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#6 |
Registered User
Join Date: Jan 2009
Location: virginia
Posts: 49
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Re: Rearend questions
Thanks. I caught my mistake after I posted it. 3 3/4 = 3.73 (roughly) Sorry for the brain fart!
Last edited by 69nova; 08-27-2010 at 01:55 PM. Reason: wrong info |
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#7 |
Registered User
Join Date: Oct 2005
Location: Overland Park, Ks.
Posts: 5,219
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Re: Rearend questions
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.
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#8 |
Registered User
Join Date: Mar 2003
Location: San Francisco, CA USA
Posts: 2,454
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Re: Rearend questions
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 |
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