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Old 04-25-2004, 08:56 PM   #1 (permalink)
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how to find out my rear end gear ratio?

I am going to order my new autometer speedo and I need to know my rear gear ratio. I put the car on stands and I marked on the tire straight down and I then turned the tire and counted how many revolutions of the u joint to one revolution of the tire and came up with 1.75 to 1. I did this a few times and came up with the same thing. What am I doing wrong? When the car was running I had 27 inch tall tires on it and a c-6 and I was taching about 3400 at 65mph. But the engine is out now and I put in a tremec 5 speed but I need a reliable way of finding this out. The other thing I noticed is that when I turn one tire the other dosen't move they turn independant of each other. I thought when you have an open rear end and turn one tire the other tire goes the other way. Any help is appreciated. Thanks
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Old 04-25-2004, 10:22 PM   #2 (permalink)
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how to find out my rear end gear ratio?

Put the trans in neutrol and turn THE DRIVESHAFT while you hold one tire still with your foot and watch the other one turn.
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Old 04-25-2004, 11:32 PM   #3 (permalink)
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how to find out my rear end gear ratio?

I took the cover off of my rear end and counted the teeth
on the ring and pinion gears. Bruce
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Old 04-26-2004, 12:30 AM   #4 (permalink)
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how to find out my rear end gear ratio?

With a posi or locked diff you can turn the tire and watch the driveshaft, with an open diff you have to turn the driveshaft and watch the tyres, i hear what fogged408 said but with my open diff i only got the right numbers when both tyres were turning very freely, making sure that the brakes weren't dragging otherwise it didn't work. I had to get a screwdriver and loosen them off a bit. by the way with your rpms i'd say its a 4.11
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Old 04-26-2004, 12:37 PM   #5 (permalink)
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how to find out my rear end gear ratio?

yeah, just divide ring teeth by pinion teeth.
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Old 04-26-2004, 02:33 PM   #6 (permalink)
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how to find out my rear end gear ratio?

I think you need to multiply by 2, since both tires are in the air. That comes out to a 3.50. Might be right.
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Old 04-26-2004, 02:38 PM   #7 (permalink)
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how to find out my rear end gear ratio?

Just re-read your post. My theory is true if the tires rotate in opposite direction, as should be with an open rear end. Not sure about your arrangement.
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Old 04-26-2004, 10:02 PM   #8 (permalink)
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how to find out my rear end gear ratio?

If you know that you were about 65 mph at 3400 rpm's, you can use a calculator probably located in some of these sites to calculate your gear ratio! As long as it is 1.1 in 3rd gear you should be able to convert your mph/rpm's and tire size to figure your gear ratio.Someone help me on this???
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Old 05-01-2004, 12:47 AM   #9 (permalink)
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how to find out my rear end gear ratio?

Quote:
On 2004-04-26 22:02, ran347nosgt wrote:
If you know that you were about 65 mph at 3400 rpm's, you can use a calculator probably located in some of these sites to calculate your gear ratio! As long as it is 1.1 in 3rd gear you should be able to convert your mph/rpm's and tire size to figure your gear ratio.Someone help me on this???
I have a spreadsheet I worked up that calculates all that - with a 27 inch tire, 65mph @ 3,400 RPMs would put you at about a 2.89, although there's slip, tire distortion and all that to enter into it, but you'll at least be in the ball park.

[ This Message was edited by: 69convert on 5/1/04 12:51pm ]
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Old 05-01-2004, 01:24 AM   #10 (permalink)
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how to find out my rear end gear ratio?

With a calculator i found on the internet, 3400 rpms at 65mph with a 27 inch diamter tyre puts you at 4.11 ratio...
anyone else have access to any calculators? just out of interest sake
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Old 05-01-2004, 06:03 AM   #11 (permalink)
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how to find out my rear end gear ratio?

Glad you posted that - I looked again this morning after some sleep and got the same result - around a 4.11 - last night at 2 am I was looking at the first gear result only!
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Old 05-01-2004, 01:58 PM   #12 (permalink)
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how to find out my rear end gear ratio?

Just a thought but if the gears are stock then you should have a tag on the bolts for your third member it is stamped with size and gear ration if not then like it was said above count the ring and pinion teeth.
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