How to determine rear axle ratio? - Chevelle Tech
Transmission & Driveline Transmissions and Differentials

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post #1 of 20 (permalink) Old Jun 6th, 20, 4:55 PM Thread Starter
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Ken
 
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How to determine rear axle ratio?

Can rear axle ration be determined by these factors?
1. At 55 mph RPM is 2250
2. Engine is a ZZ4 350 V8
3. Transmission is TH350

67 Chevelle Malibu, ZZ4 350 crate engine, TH350, FiTech EFI, Vintage Air, Dakota HDX dash, FlowMaster exhaust, Digi-Tails tail lights and front marker lights, 17" Rally style wheels, Redline tires
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post #2 of 20 (permalink) Old Jun 6th, 20, 4:57 PM
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re: How to determine rear axle ratio?

Quote:
Originally Posted by darksky View Post
Can rear axle ration be determined by these factors?
1. At 55 mph RPM is 2250
2. Engine is a ZZ4 350 V8
3. Transmission is TH350
What gear are you in at 55 ? Third I'm assuming. And what is rear tire size ?

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post #3 of 20 (permalink) Old Jun 6th, 20, 5:51 PM
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re: How to determine rear axle ratio?

Is speedo correct?

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post #4 of 20 (permalink) Old Jun 6th, 20, 6:02 PM
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re: How to determine rear axle ratio?

1967 Chevelle Rear Axle Codes

Gear ratios vary, but the Turbo 350 typically has a first gear ratio of 2.52 to 1.00; 1.52 to 1.00 for second; direct drive in third; and 2.07 to 1.00 in reverse.


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post #5 of 20 (permalink) Old Jun 6th, 20, 7:27 PM
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re: How to determine rear axle ratio?

What is your tire height ??

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post #6 of 20 (permalink) Old Jun 6th, 20, 7:33 PM
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re: How to determine rear axle ratio?

If you have 26 inch tires, you have a 3.07 ....

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post #7 of 20 (permalink) Old Jun 7th, 20, 1:42 AM Thread Starter
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re: How to determine rear axle ratio?

Quote:
Originally Posted by Alan F View Post
Is speedo correct?
Yes

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post #8 of 20 (permalink) Old Jun 7th, 20, 1:44 AM Thread Starter
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re: How to determine rear axle ratio?

Quote:
Originally Posted by howsad4me View Post
What gear are you in at 55 ? Third I'm assuming. And what is rear tire size ?

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In third gear at 55mph. Tire size: 255-50R17

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post #9 of 20 (permalink) Old Jun 7th, 20, 3:09 AM
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Re: How to determine rear axle ratio?

Do you have a posi rear end? If not, then use your passenger side rear wheel to determine the gear ratio in the rear. Get the rear end of the car up in the air with both rear tires off the ground safely resting the frame or the axle tubes on two jack stands with wheel chocks under the front tires to prevent the car from rolling. Either use a piece of masking tape or some white-out paint on the side of the driveshaft. You can also do the same on the passenger side rear tire as well. Shift the transmission into neutral with the emergency brake off.

Watching the mark you've placed on the driveshaft, count exactly how many turns the driveshaft makes while turning the rear wheel by and one full revolution. If the driveshaft turns 3 1/2 turns, you have 3.55 gears in the rear. If it turns a hair past 4 turns, you have 4.10 rear gears. If it's just a hair over 3 turns, you have 3.07 gears, etc. etc.

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post #10 of 20 (permalink) Old Jun 7th, 20, 6:43 AM
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Re: How to determine rear axle ratio?

Non-Posi, you raise one wheel off the ground.... Posi, both wheels ....
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post #11 of 20 (permalink) Old Jun 7th, 20, 7:21 AM
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Re: How to determine rear axle ratio?

According to this Calculator it is a 3.36/3.31 Rearend Ratio

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post #12 of 20 (permalink) Old Jun 7th, 20, 11:49 AM
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Re: How to determine rear axle ratio?

Quote:
Originally Posted by thebeast View Post
Non-Posi, you raise one wheel off the ground.... Posi, both wheels ....
And if you do it non-posi, one tire off the ground, one tire fixed...you have to do two tire revolutions to get actual rear gear ratio at the driveshaft count, due to the fact you are rotating through the spider and axle side gears too, it cuts everything in half.

See this all the time where guys jack up one side, spin the tire once, get 1-1/2 turns on the driveshaft and think they only have 1.50 gears....they actually have 3.00 or 3.07/3.08 depending on what manufacturers rear axle they are using.

With the tire height calculator saying the 255/50-17 is 27" tall, and figuring 5% torque converter slip I got a rear gear about 3.07/3.08 for the OP's example.
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post #13 of 20 (permalink) Old Jun 7th, 20, 2:50 PM
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Re: How to determine rear axle ratio?

Quote:
Originally Posted by thebeast View Post
Non-Posi, you raise one wheel off the ground.... Posi, both wheels ....
Quote:
Originally Posted by Ericnova72 View Post
And if you do it non-posi, one tire off the ground, one tire fixed...you have to do two tire revolutions to get actual rear gear ratio at the driveshaft count, due to the fact you are rotating through the spider and axle side gears too, it cuts everything in half.

See this all the time where guys jack up one side, spin the tire once, get 1-1/2 turns on the driveshaft and think they only have 1.50 gears....they actually have 3.00 or 3.07/3.08 depending on what manufacturers rear axle they are using.

With the tire height calculator saying the 255/50-17 is 27" tall, and figuring 5% torque converter slip I got a rear gear about 3.07/3.08 for the OP's example.
Thanks for the correction guys.

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post #14 of 20 (permalink) Old Jun 7th, 20, 3:40 PM
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Re: How to determine rear axle ratio?

My 3.31's turn 2650 at 60 MPH. In the past my 3.55 turned about 2800 and my 3.73 did 3000 at 60. I suspect if you have a 10 bolt you have 3.08's and 3.07 for a 12 bolt which spin about 2400 at 60. That is with approx 26.5 tire height. Your tires are about 27.1 inches in diameter which give you lower RPM then what I stated but not a lot.
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post #15 of 20 (permalink) Old Jun 7th, 20, 5:06 PM Thread Starter
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Re: How to determine rear axle ratio?

OP here... Thanks for all the suggestions, but I don't have a floor jack or jack stands. Was just curious if rear end ratio could be determined with the info I provided.

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