OT rear end ratio

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Model T Ford Forum: Forum 2012: OT rear end ratio
Top of pagePrevious messageNext messageBottom of page Link to this message  By Optics Guy on Wednesday, July 11, 2012 - 05:40 pm:

How to determine the rear end gear ratio? Not for a T but for a 56 Chevy. My friend rotates the drive shaft 2 times and the rear wheel rotates 1.16 times. This does not give me a ratio that comes close to the standard rear end ratios found in these cars. So, what are we (he) doing wrong or better yet, how to determine the gear ratio???


Top of pagePrevious messageNext messageBottom of page Link to this message  By Layden Butler on Wednesday, July 11, 2012 - 07:10 pm:

Least confusing and safest way on a vehicle with exposed driveshaft. Block securely ahead and behind 3 tires and jack up one of the rears just off the ground. Handbrake off and transmission out of gear.( sounds like you have done this part) Rotate the driveshaft counting the number of revolutions until the wheel has made exactly 2 complete revolutions. The driveshaft turns will be the commonly used ratio number examples: 4.11 or 3.76 or 2.73 etc.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Roar Sand on Wednesday, July 11, 2012 - 07:38 pm:

If the 1.16 is accurate, divide by four (2 for the two turns and 2 for one wheel on the ground) and take the inverse of that and you get 3.45:1, which at least is within a reasonable range.
Roar


Top of pagePrevious messageNext messageBottom of page Link to this message  By Optics Guy on Wednesday, July 11, 2012 - 11:14 pm:

I am a little confused. Will go out to look at my friends car chassis tomorrow and do an accurate count doing the method described here.

Thank you.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Ted Dumas on Thursday, July 12, 2012 - 12:07 am:

With one wheel off the ground, the differential cuts the ration in half. So two turns gave you 1.16 turns of the wheel. One turn would give you 0.58 turns of the wheel. Your ratio is then 2*(1/.58) or 3.45 as Roar stated above.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Doug Money - Braidwood, IL on Thursday, July 12, 2012 - 08:39 am:

Not to be picky, but if this is the normal Chevy rear axle, I believe it should be 1.17 not 1.16.


Top of pagePrevious messageNext messageBottom of page Link to this message  By John W. Oder - Houston, Texas on Thursday, July 12, 2012 - 12:45 pm:

Jim's 220HP Bel Air was bought new in '57 with a 3.55:1. It was three on the tree and would scoot along well. :-)


Top of pagePrevious messageNext messageBottom of page Link to this message  By Optics Guy on Thursday, July 12, 2012 - 04:40 pm:

Ok, went out to my friends house and did an accurate count:
2 turns of the wheel gave 3.5 revolutions of the drive shaft.

This ratio gives me 1.75:1 so how do you figure out the supposed "real" ratio? Times 2 would give me 3.50 ??????????????????


Top of pagePrevious messageNext messageBottom of page Link to this message  By Doug Money - Braidwood, IL on Thursday, July 12, 2012 - 06:40 pm:

That equates to the 2.29 gear ratio. Nice highway ratio for gas mileage.

2 X (1/.875)= 2.29


Top of pagePrevious messageNext messageBottom of page Link to this message  By Layden Butler on Thursday, July 12, 2012 - 07:46 pm:

Reread my last sentence. 3.5 is your rear end ratio. Due to accuracy of measurements it is probably the 3.55 that John spoke of.


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