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Old 11-14-2003, 11:31 PM
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How to tell gear ratio???

I have a 78 El Camino, SB 350 V-8, B&M 350 transmission, but am wondering (since I bought the car used) how can I tell what the gear ratio is on the rearend? I don't know if it is stock or if it has been replaced. Is there a way I can check this myself?

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Old 11-15-2003, 03:44 AM
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Most GM rears have a code stamped on them. you can go to your local book or part store and look in the haynes manual for your vehicle and copy the codes, the code will tell what ratio it is, and tell if it`s a posi.
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Old 11-15-2003, 08:48 PM
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Where exactly would I like for this stamped number???
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Old 11-15-2003, 09:35 PM
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A quick way to check is to jack up the right hand wheel so you can crawl under the car to work. Put the car on a jackstand and block the tires so you can put the trans in neutral and not have it roll on your head. Make a mark on the pinion and pinion snout so you can count the number of revolutions of the driveshaft. Make a mark on the inside of the tire or wheel. Now, with the car in neutral, spin the driveshaft by hand one revolution. Count the number of times the rear wheel goes around. If it goes around about 2 3/4 times you have a set of 2:73's. A little over 3 times would be 3:08's. Almost 3 1/2 revs, but not quite, means you have 3.42's. See how it works? It'll get you in the ball park...
Mark

Notice I did state to jack up the right hand side only....
Mark

Last edited by astroracer; 11-17-2003 at 04:28 AM.
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Old 11-15-2003, 10:24 PM
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If you jack up one wheel, you must multiply your result by two, then see what was available in your rear end that comes close. Jacking up two wheels works well (and in fact is necessary) with posi, but with a single-track, it's not reliable because the wheels will sometimes turn in opposite directions.

1-1/4 turns=around 2.5 (2.41, 2.56)
1-1/3 turns=2.7 (2.73 or thereabouts)
1-1/2 turns=3.0 or 3.08
1-3/4 turns=3.5 (3.55 or 3.42)

etc.
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Old 11-15-2003, 10:35 PM
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With an open diff (one wheel peel) make sure one wheel is held in place (have a buddy hold it, or leave it on the ground). Turn the other wheel two revolutions while counting drive line revolutions. Oh yeah, car in neutral.
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Old 11-15-2003, 10:37 PM
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Just what I was thinking Jim You have to remember those spider gears
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Old 11-17-2003, 09:51 AM
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The messy way is to remove the back cover. On the ring youŽll see some numbers. The last 2 pair of them (they stand separate) will give you the ratio. Divide the larger number (for ex. 41) with the smaller number (ex. 11) and you have the ratio; in this case it would be 3.73.
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Old 11-17-2003, 10:01 AM
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What would a good gear ratio be?
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Old 11-17-2003, 10:35 AM
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Quote:
Originally posted by maddmaxx
What would a good gear ratio be?
Eh... 3.73. 41/11=3.73

Edit: My apologies. I misread your post.
Myself, IŽm going for 3.42. It should give me good acceleration but still keep cruising rpm at a fairly low level. I have 245/50/14 tires.

Last edited by Pianoman; 11-17-2003 at 01:40 PM.
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Old 11-17-2003, 01:01 PM
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Quote:
Originally posted by maddmaxx
What would a good gear ratio be?
That all depends on your tire size, vehicle weight, your engine's power band and what you plan on doing with your ride. In a 3000lb+ car, 3.73's in most cases are perfect for street/strip applications. 3.55's are good for performance cruising. 4.11's are most generally reserved more for the quarter mile "Sportsman." Once you dip into the 5.??'s, the eight mile is just about all that ratio is good for, that is, unless you're running an engine that can turn 8 grand+ and still pull hard without coming apart.
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Old 11-26-2003, 01:05 AM
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I agree with pianoman, only for sure way. Gives you a chance check it out.
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Old 11-26-2003, 09:16 AM
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Talking

I agree....take the cover off and count the teeth. It doesn't hurt to change the fluid which is the most neglected fluid in a vehicle. Most GM's only need a couple quarts. If a "posi" be sure to use the proper fluid.
__________________
Tom
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Old 11-26-2003, 11:50 AM
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Quote:
Originally posted by dollybud
I agree....take the cover off and count the teeth. It doesn't hurt to change the fluid which is the most neglected fluid in a vehicle. Most GM's only need a couple quarts. If a "posi" be sure to use the proper fluid.
May I recommend Red Line? No additives needed
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Old 11-26-2003, 06:07 PM
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HEY MADD: I can read it in your eyes! You are wanting to put in a lower ratio to get a quicker take off. My son changed to a lower ration in his Impala SS and it will melt the street BUT at higher speeds the engine is running much higher RPM's and his gas milage went from bad to worse....Cannot have it all....DAVE
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