Gears? -

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post #1 of 6 Old 04-10-2013, 10:34 AM Thread Starter
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1985 J-Series Truck 
Join Date: Apr 2012
Location: burlington
Posts: 95

Does anyone know what the stock gears were in 1985 J-10? How to identify what I have, and the best gearing to run with 35's? Never really messed with this kind of stuff, and don't know what has been done to this truck. Besides doing some hunting and fishing I am mostly going to be driving street and highway. Sin I know.

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post #2 of 6 Old 04-10-2013, 10:50 AM
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1976 J-Series Truck 
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there should be a tag on the diff, held on by th bolts for the diff cover. That should have your ratio on it. There should also be one on the front diff, if the one on the rear is missing. It may just say '3.54' or whatever, or it may be like '43 16' and you get to do the math yourself.

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post #3 of 6 Old 04-10-2013, 03:48 PM
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1975 CJ6 
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An '85 should have an AMC Corporate axle in the rear (the M23). Look here for the axle codes:

The front axle should have a tag, as mentioned. Likely it's 3.31 or 2.73, by 1985.

Re "the best" gearing, for off-pavement the higher the number the better. But you may have to compromise to get acceptable engine speed on the highway. Decide what engine speed and highway speed you want, then use one of the online calculators to find an acceptable gear ratio. - for example, 35s at 65 mph and 2500 RPM is about a 4.10 axle ratio.

1) '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS, hubcaps. 2) '82 J20 360/T18/NP208/3.73, KOs, 7600 GVWR 3) '75 CJ-6, 304/T15, PS, BFG KM2s, soft top 4) '95 VW Golf Sport 2000cc 2D
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post #4 of 6 Old 04-11-2013, 09:20 AM Thread Starter
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1985 J-Series Truck 
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What if someone has changed them already? Is there a way to tell what they replaced them with? Are the gears themselves marked? Or do you just count the teeth?
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post #5 of 6 Old 04-11-2013, 10:42 AM
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The ring gear will have a stamp on the outer circumference that gives the tooth counts. Do the math and it will tell you the ratio. You'll see something like 41-10 (4.10 ratio) or 43-13 (3.31).
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post #6 of 6 Old 04-12-2013, 11:33 AM Thread Starter
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1985 J-Series Truck 
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Location: burlington
Posts: 95
Thanks everyone!
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