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: www.ericthecarg...
Visit www.5speeds.com for more helpful information from Paul Cangialosi.
Pauls book on rebuilding manual transmissions: www.5speeds.com...
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Paul's KZbin Channel: / gearworldtv
Special thanks to Pull-a-Part for the differential used in this video: www.pullapart.com
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T-5 Transmission Rebuild with Paul Cangialosi: Link posted when video becomes available.
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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.

[@351WINCHESTER]

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.

[@DIYAutoHomeschool]

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.

[@usmale47374]

Paul seems like a knowledgeable man. And I know where he's going with the final drive ratio in a car with OD. I had a '70 Mach I with a 428 cid and a 4-speed tranny with a 411 rear end. It would get to 100 mph lightning fast, but it topped out soon after. Had it had OD tranny, that would been a good combination, so consider Paul's observations and advice if you're primarily interested in your car's sprint abilities.

[@PHILLY214]

all stock?

[@AimlessMoto]

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.

[@andrewcbro]

this series with Paul is amazing!!

[@GearBoxVideo]

+Andrew Brown Much appreciated thanks.

[@InternetDude]

Ford never made a posi-traction differentials but GM did. Ford made TractionLok limited slip differentials.

[@ericthecarguy]

+InternetDude You are correct, but I think you got my meaning.

[@winningoO]

I like the AMSOil boxes in the background!

[@chevymuddigger19]

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

[@45von]

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....

[@8avexp]

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.

[@SouthMainAuto]

Hey Eric I have more of YT question. I see your video has been up for only 3 hours, has 123 views and 300 likes? How does that work? Is there another platform you can display videos on that allows the user to "thumbs up" the video but not have it show up as a view. Just curious, hate to be missing out on something :)

[@SouthMainAuto]

+Rex Overbey ahhhhh ok ok ok I see now ha ha, my channel doesn't get views like the car master here so that is why I have never reckonised it before, always wondered though. ..thanks

[@ericthecarguy]

+South Main Auto Repair When you first post a YT video the view count isn't accurate for several hours. BTW I think your channel is doing just fine. Keep up the good work. Also, it's funny, last month I shot a video on alignments. Mine won't air till sometime next year, but I'm sure I'll get a lot of flack because you've already covered it. :)

[@SouthMainAuto]

+EricTheCarGuy got ya... bahhh when we all work on the same crap day in day out, eventually were all gonna cover the same stuff ya know. I mean unless you get a futuristic flying hover car in before I do lol

[@ericthecarguy]

+South Main Auto Repair No worries. You and I are good, but you know how some of the fans can get about that kind of thing.

[@jakebopp6009]

+EricTheCarGuy NO INTER-CHANNEL MINGLING. YOU ARE SWORN ENEMIES. I watch you both daily, backlog or new episodes. You both sell me tools I don't need. +South Main Auto Repair and his damn Astro Mini, and ETGC with his damned Knipex products... Thanks for the videos, regardless. Hours of entertainment. My girlfriend hates you both.

[@morganmckinniss4653]

This works with an open differential by using two rotations of the axle hub (drum brake in this case) instead of one.

[@rinu123]

You can do the same with an open diff by simply holding the other side so it doesn't move.

[@ericthecarguy]

+rinu123 Nope. Then the other side will spin twice as much.

[@IIGrayfoxII]

+EricTheCarGuy Then couldnt you divide by 2

[@tmmtmm]

+EricTheCarGuy then just divide the resulting ratio by 2 and voila. Or using the method shown, count two turns of the hub instead of 1.

[@PHILLY214]

it don't matter...if it's open all the power goes to the inside tire or one with less traction.....a limited slip will do the same

[@patw52pb1]

+hittin-low_150db+ Try it and see that you are mistaken.

[@teddymeredith2565]

You can do that on an open diff if you hold one axleshaft

[@cgomezgti]

I miss those repair videos

[@SomeGuyInSandy]

"...put that on a little video too." Nice Paul, lol =)

[@MorbidThrasher666]

I had 4.10's in my 92 mustang and it was auto then converted to stick and i barely had 1st gear after but man was that a fun neck breaker. My current 94 gt still has stock 3.08 gears but will upgrade later on to 3.73

[@ArchimedeanEye]

If 4th gear is 1:1 and 5th: is 0.6... why not go with either a 3.73, 3.90, or even 4.11?

[@peterbiltxr379]

+ArchimedeanEye i was thinkin 3.55 or 3.73

[@melodicdreamer72]

+ArchimedeanEye I would also narrow it right down to 3.55 myself as it sounds ETCG has - from experience. Since the 5.0 liter going into the Fairmont has a relatively low red line connected to a manual transmission with standard diameter tires, the 3.55 will keep the car at a good point in the power band throughout the gears. With a 3.73, 1st gear from a dead stop to red line will be really fast and tire spin becomes an issue. On the other hand, if it had your basic automatic transmission (why would one do that?) the 3.73s are a good way to go - in OD you'll still be able to keep the car under 3000 RPMs on pretty much any highway as well. On the other other-hand, if there was a 4.6 / 32 valve motor going in with the manual, the 3.73s move things to a happy place. Much lower gears are not very good unless you are going for a mostly dedicated drag car with big slicks and heavy suspension and drive-train modifications to handle the load. Just my two cents.

[@GearBoxVideo]

+ArchimedeanEye +melodicdreamer72 You have to keep in mind the whole package. The transmission has a 3.35 1st gear. Typically that is too much first gear to run with a 3.73 or 4.11. Actually most of the Mustang 5.0 had 3.08 and 2.73 ratios with that gear. The issue becomes how the engine is cammed and where the torque curve of the engine sits. From what I discussed with eric, the way that engine is cammed, with a 2.73 iut would choke off the line. The overdrive ratio on the final build of the transmission ended up at a .73.

[@ericthecarguy]

+ArchimedeanEye We found out my transmission is actually for a 6 cylinder. Overdrive in the 6 cylinder is .73. Therefore I'm going with 3:55.

[@realmaindrianpace]

+EricTheCarGuy Is that six cyl trans a world class (roller bearings)? if not, a WC is about mandatory for a healthy small block.

[@kennedy796]

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?

[@cobra454tim]

Another great video, Thanks guys !

[@screamineagle88]

Ok, here's a question. You were saying that you can only preform this test on a posi rear. Is it because it spins both wheels when you turn the pinion? If so, then could you do this test on an open diff if you blocked (or locked) the other drum (wheel) side?

[@joecrane7873]

If you can lock one side of any open axle, would the same process work?

[@TorqueEffect]

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.

[@hydroboostoxiaction6271]

Eric you clean up well

[@davidrathburn6963]

I have a 77 vette and the person I bought it from said the rear end ratio was 3:70. I wanted to bring this down for thruway driving. A 3:08 is what I decided, I know no jackrabbit starts but I'm beyond that at 72. Took it to a shop and they did this quick check for the ratio and found it to be a 3:08. My question is this, can this quick check be inaccurate? I dont see how but the guy insists their wrong. It's a posi rearend and no tags on it to tell me what it is. Hate to open it up to be sure.

[@kodypippin6770]

don't fear the gear, I'd rather have too much gearing than too little

[@bigandfluffy2000]

judging withing the first 10 seconds, if RCR taught me anything its a for 9inch..

[@daytona2134]

2:34 he was getting a tude haha

[@Courage68]

3.73 with a 0.6 5th should be 2k rpm at 70mph , Thats what i had in mine.

[@hollywoods7295]

2:73

[@deronborghi5929]

So, I should still be able to get A close enough number with an open diff as long as I chalk one side, is that accurate?

[@jeb721]

Hey Eric, is that a J series behind you? 3.5? Going in something? What's it from?

[@julianoliva2850]

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?

[@MrDelow11]

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

[@TheJhaley12]

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

[@josh33025]

So he's one of those guys that when you talk to him he says so many numbers that if you don't understand it you're lost.

[@XHikotheproX]

2.76? does he mean 2.73?

[@GearBoxVideo]

+Patrick H Yes

[@TGsrao1989]

Are you not going for the 5 lug disc conversion?

[@ericthecarguy]

+TGsrao1989 Yes

[@jeffwebber4816]

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

[@dhanarajsingh7380]

Hello Sir I want asked that is many teeth is strong for carrier or less teeth? Please reply

[@xMLK3Y]

whats with the j series under the table?

[@alnbaba]

My ford has 410 gears at 70 in 5th gear I'm turning 3000rpm.

[@fishyer777]

Where did the J series engine in the background come from?

[@ericthecarguy]

+Chris Phillips It's replacement will be featured in an upcoming series when I get the opportunity to complete the edits.

[@fishyer777]

+EricTheCarGuy Can not wait. Have a 6th gen accord with the J30 and absolutely love the engine. Just wish I could say the same for the transmission

[@ericthecarguy]

+Chris Phillips Yea, those transmissions weren't the best. You're much better off with a manual trans for those years.

[@F6HemiCharger]

Aww. Posi-Track? I think you mean Traction Lok

[@PHILLY214]

limited slip

[@F6HemiCharger]

+hittin-low_150db+ Chevy: Posi-Trac, Ford: Traction-Lok, Mopar: Sure-Grip all are limited slip.

[@wilsonautomotive]

+F6HemiCharger They're all just a limited slip. Don't get too caught up in the names. They're just irrelevant marketing things anyway.

[@F6HemiCharger]

+Mr Yota I know. Call it what it is then, limited slip. I'm just making a point, if someone is going to use a brand specific name for it, it should be used correctly.

[@f4cphantom2]

I just google it...

[@Tat2Dragons]

Well you can't always do that.

[@estrelladelnorte1308]

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.

[@GearBoxVideo]

+estrelladelnorte1308 start at clip time 1:06 The amount of turns of the pinion to 1 turn of the drum is your approximate ratio. I thought it was pretty obvious. Most people got it. Is it that you did not understand what the word "ratio" mean't?

[@PHILLY214]

and wouldn't that be different if the drums were a wider diameters. ...just take the cover off and find out for your self

[@patw52pb1]

+hittin-low_150db+ No. One revolution is still 360 degrees regardless of the diameter. Actually not necessary to remove the cover to determine the ratio, since as Paul stated the ring and pinion gear sets are known standard values it makes it relatively easy and accurate to determine the ratio by the method shown in the video.

[@PHILLY214]

+patw52pb1 pretty sure it would be diff...it's bigger takes more to do 360 degrees ...and not very accurate...I'll stick to my 3:73s

[@GearBoxVideo]

+hittin-low_150db+ It can be confusing but RPM is RPM. Forget about the drum, think about the tire bolted to that drum. The outer surface speed will be different depending on the circumference of the tire. That is why speedometer gears are different when someone is using a 25" diameter tire versus a 30" tire. The tire will still make one revolution, but the one with the larger circumference will cover more distance per revolution than the smaller one, thus requiring a different speedometer gear set combination to achieve correct calibration.

[@danielwarren6845]

+hittin-low_150db+ no it wouldn't make a difference. The shaft is being turned, not the drum. The speed would be different but not the amount of turns.