A Quick Easy Way To Determine Axle Ratio -EricTheCarGuy

Paul Cangialosi came to my shop to help me do the rebuild on the T5 transmission that’s going into my 1979 Ford Fairmont. We were at the point of trying to figure out what speedometer gear to use when he asked what the gear ratio of the rear end I was going to use was. I said I didn’t know and he said he had a quick way of figuring out what I had.

This will only work on differentials with posi traction. That means that both wheels spin in the same direction when the pinion flange is rotated. If one wheel rotates one way and the opposite wheel the other, this method will not work. If however you have a posi traction rear end like I do, you’re good to go.

I can’t thank Paul Cangialosi enough for his time in the shop that day. I learned a great deal and hope to pass along that knowledge to you.

Camera: Brian Kast

Thanks for watching!

The best place for answers to your automotive questions: http://www.ericthecarguy.com

Visit http://www.5speeds.com for more helpful information from Paul Cangialosi.

Pauls book on rebuilding manual transmissions: http://www.5speeds.com/cart/index.php?route=product/product&product_id=33

Electronic version of Paul’s book: https://books.google.com/books/about/How_to_Rebuild_and_Modify_High_Performan.html?id=OexeE4mTsZYC

Paul’s YouTube Channel: https://www.youtube.com/user/GearWorldTV

Special thanks to Pull-a-Part for the differential used in this video: http://www.pullapart.com

Related videos

What Speedometer Gear is Right For You?: https://www.youtube.com/watch?v=fMEe5SySuxA

How a Manual Transmission Works: https://www.youtube.com/watch?v=d9clD8ZL8xA

A Brief History of the T-5 Transmission: https://www.youtube.com/watch?v=oVPAeTnRdA0

T-5 Transmission Rebuild with Paul Cangialosi: Link posted when video becomes available.

Discussion about this video: http://www.ericthecarguy.com/kunena/18-The-EricTheCarGuy-Video-Forum/61098-a-quick-easy-way-to-determine-axle-ratio#152938

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Due to factors beyond the control of EricTheCarGuy, it cannot guarantee against unauthorized modifications of this information, or improper use of this information.  EricTheCarGuy assumes no liability for property damage or injury incurred as a result of any of the information contained in this video. EricTheCarGuy recommends safe practices when working with power tools, automotive lifts, lifting tools, jack stands, electrical equipment, blunt instruments, chemicals, lubricants, or any other tools or equipment seen or implied in this video.  Due to factors beyond the control of EricTheCarGuy, no information contained in this video shall create any express or implied warranty or guarantee of any particular result.  Any injury, damage or loss that may result from improper use of these tools, equipment, or the information contained in this video is the sole responsibility of the user and not EricTheCarGuy.


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24 thoughts on “A Quick Easy Way To Determine Axle Ratio -EricTheCarGuy

  1. Nice video I'm looking to find out what are the gear ratios on a differential and for what kind of use are designed? Any one can say something about?
    Cause upgraded my differential to posi but dont know the gears ratio and what that means lowers or highers?

  2. Very similar to the way I figured out the axle ratio on my '69 AMX – only I raised the rear end and spun the left rear wheel while watching the drive shaft.. Mine has a 3.54 posi – with a T10 4-speed. And I put Amsoil Signature Series 10W30 oil in my 390.

  3. How would he be able to figure out what ratio he need for his car and why wouldn’t he be able to use that ratio????? I need help understanding bc I wanna change mines on my car

  4. I'm thinking of putting a M-20 in my 55 210. It supposedly has 3:55 gears in the rear. (I say supposedly because the guy I got it from lies so much I really don't know, I believe it might be a 4:10 because it's so low) it also has 15 inch tires, a 350/300 hp engine in it. How will my top end speeds be if I do this? Someone please answer this because I really need the help. Thanks

  5. I'm a truck guy from way back.  I learned quick to get the lowest rear end ratio made so I could put taller tires to increase ground clearance.  Most of the time 4:10 was the sweet spot.  Nowdays with the newer transmissions you don't need to go as low, but it still helps if you are going to use your truck.

  6. Maybe you can help me!
    I have a 91 toyota pickup 4cy 22re. i had a whine when i coast, looked under pinion was leaking and driveshaft would wobble. got it fixed, a week later same thing.. whats causing it to break? bad u joint? i also have vibration at 45 and 65 but 70 is smooth, im thinking thats my tires but not sure..
    thank you

  7. couldnt you do this with a open differential? i mean lock one hub to make it not move, turn the pinion once, then divide what you find in half?

  8. ok ….. the best way is ask paul or erik. beacuse all i hear was "1,2, ..i thought it was a 3.08 that was my guess bla bla and this is a 2.76 and bla bla" you never explained the math behind the turns did you? all i learned was that he has a book and and his name is paul.

  9. Am I right in guessing that came from Upullandpay in Cincinnati? Love that yard. Got allot of good parts from them, and they are the cheapest around.

  10. This same technique CAN be done with a standard open differential, you have to just prevent one side from moving. You can do it with both wheels turning too, but it takes simple math. You'd have to divide tire rotations (t) by yolk rotations (y)… and only count when both marks are touching. Then add them, and divide by however many you performed. For example. 3y1t = 3, 6y2t = 3, 9y3t = 3, (3+3+3)/3 = 3.00 ratio. Another example:  5y2t = 2.5, 8y3t = 2.67, 12y4t = 3, (2.5+2.67+3)/3 = 2.72 ratio. The more rotations you do… the more accurate.

  11. Hey Eric, I'm sure other people have mentioned it, but this same trick is workable on an open differential. Lift the vehicle (or place the axle assembly on jack stands, mark the drums the same way he did, just on both sides, as well as the pinion flange. Then, have someone hold one side perfectly still while you turn the other side 1 full revolution until the marks line up again, then hold that side while they turn the other side the same full turn. Count how many times the pinion flange turned and you have your answer. Easy and quick way to get differential gearing.

  12. if it was NOT a posi… You would simply Stop one wheel and count the number of turns on pinion to cause 2 turns of the not held wheel….

  13. hey Eric. I got 98 svt cobra with t-45 and 5th gear is .67 and I have 4.10 and going 75 mph my rpms is 3000. just giving little hints of a lower gear. great videos and can't wait for more of the Fairmont

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