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Old 04-17-2008, 06:06 AM
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speedo and rear end question

im putting a rear end from a 85 monte carlo ss in my 85 el camino. the monte rear end has a 3.73 ratio and whats in my elky is 2.41 ratio. would i have to do some changes to my speedo or would it read the same with either rear end..if my elky speedo reads wrong with the 3.73 then would it be right if i were to use the speedo from the doner monte..the monte rear is going behind a th350 for now till i can get my hands on a 200r4 ..thanks
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Old 04-17-2008, 07:47 AM
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No, the speedos are all geared the same. You need the drive gear off of the cable to calibrate the speedometer.

The formula for getting the right gear:

#Drive Teeth x Axle ratio x Tire Revs per mile
1001 = # driven Teeth

"drive teeth"= number of teeth on the gear INSIDE the trans that turns the speedo
"driven teeth"= # of teeth on plastic gear on cable
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Old 04-17-2008, 08:05 AM
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Quote:
Originally Posted by sharpe427
No, the speedos are all geared the same. You need the drive gear off of the cable to calibrate the speedometer.

The formula for getting the right gear:

#Drive Teeth x Axle ratio x Tire Revs per mile
1001 = # driven Teeth

"drive teeth"= number of teeth on the gear INSIDE the trans that turns the speedo
"driven teeth"= # of teeth on plastic gear on cable

I am sure you meant to post the speed-O are not all geared the same...



both gears may need to be changed.

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Old 04-17-2008, 09:09 AM
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should i get the speedometer and cable for the donor car(monte ss)
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Old 04-18-2008, 03:56 AM
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well i thought i would get the driven grear from the donor car but turns out he has got the trans sold so im left trying to figure what i need..could i just go to a parts store and tell them i need a speedo driven grear for a 85 monte carlo ss .. would that work..this is what im going to have in the car..th350 trans. rear end with 3.73 gear ratio and the wheels are 14"
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Old 04-22-2008, 02:06 AM
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There's a tire diameter calculator at http://tciauto.com/Products/TechInfo/speedo_gears.asp . Take that diameter and your rear end ratio to http://novaresource.org/speedo.htm and use the formula to find the speedometer gear ratio you need. Look at the chart and see the various gear combinations that will give you that ratio, or something real close.

Count the teeth on your driven gear and see if it fits one of the combos, and if it does, buy the corresponding drive gear and install it on the tailshaft of your T-350. If not, you get to buy both gears. They're fairly cheap at the Chevy place.

You can do the same with the 200-4R when the time comes, but the drive gear is on the governor. http://www.transmissioncenter.net/2004RTIP.htm has a chart that will save you some math.
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Old 04-22-2008, 02:39 AM
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hey thanks for the info..im sure i'll get it figured out now..i want to get that part right before i put the rear under the car..
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Old 04-22-2008, 05:52 AM
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dave,

once the rear is in the car, find a cop with a radar gun and explain you are re-calibrating the speedo gears.....

have him "shoot you" with the radar gun while holding the rpms steady at 1,000 and 2,000 and 3,000....
use those values in the TCI calculator to account for your converter % slippage....

once you know the exact mph per rpms you don't really need a correct speedo...
EX:
(I really lucked out) with my 24" tires and 3.55 gears the radar gun showed each 1,000rpms= exactly 20mph on my car so when my tach reads 3,000 I am doing 60 on the button!

how's the starter project going?
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Old 04-22-2008, 06:12 AM
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dave,

here's a super simple to use graph table of tire sizes to gears ratio=55mph ballpark depending on convertor slippage

http://www.amxfiles.com/resource/tech/rpmvs.html
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Old 04-22-2008, 07:10 AM
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thanks for the link.... as far as starter is concerned im having some #2guage cables built. im thinking my stock cheap light gauge cables are causing my starter problems...
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Old 04-22-2008, 07:19 AM
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Quote:
Originally Posted by Crosley
I am sure you meant to post the speed-O are not all geared the same...



both gears may need to be changed.

Almost all speedometers are geared to 60 MPH at 1000 revs...haven't seen one different since 1964--and that was in an English car. The trans drive gears may be different. And the internal drive gear would only need to be counted for teeth. To avoid dis-assembly of the trans (in worst case) a reducer/adapter can be used externally if the need arises. Usually, count the number of internal teeth, do the simple math and install the correct driven gear to get the right speed indicated. I disagree on not needing a correct speedo...good for the odometer (insurance) and good for wallet.

Last edited by sharpe427; 04-22-2008 at 07:27 AM.
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Old 04-22-2008, 07:25 AM
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GM cable speedos are calibrated at 2000 tire revs/mile. The electronic ones are 2000 pulses/mile.
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Old 04-22-2008, 07:31 AM
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just to be clear...
that table chart is for a "stick shift" car which has no slippage

yup,
because copper wire is expensive and pvc insulation is dirt cheap...
they "cheat" on elcheapo cables and just use a thicker insulation jacket so it "looks" to be the same wire size as a good cable...
or they use thicker wire strands versus more thinner wire strands per cable size..."fine strand" is better
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