Differential ratio?

Btrain
Oct 21, 2020

Rank 0

Oct 21, 2020

I have a 1971 302 4 speed, 25" tires. Absolutely no tag/ markings on Ford 9". Trying to figure out what differential gearing I have. In 2 Hi, at 55 mph I'm turning 3k rpm. Any guesses?
Last edited by a moderator: Oct 21, 2020
Moderator

Moab or bust!!!

Oct 21, 2020

#1
I have a 1971 302 4 speed, 25" tires. Absolutely no tag/ markings on Ford 9". Trying to figure out what differential gearing I have. In 2 Hi, at 55 mph I'm turning 3k rpm. Any guesses?
I know there probably is an easier way to figure it out but I've always used the "manual" way of checking.
If you jack up the rear end so both tires are off the ground, you can look at the ratio of how many turns your driveshaft turns compared to 1 full rotation of the tire.
Place your truck in neutral and slowly rotate your tires while watching the driveshaft. Probably a good idea to mark the driveshaft with a clear chalk mark.
If you rotate your tires 1 full turn and your driveshaft rotates just past 4 turns, then you have 4.10 gears.
Sorry, I don't know what the popular rear end ratios are for a '71 plus they might not be stock anyways.

Hope this helps.
2022 Bronco Badlands, 4D, Hot Pepper Red, 2.7, Squatch, Lux, and MIC.
7/13/2020 10:04pm reservation. Blend 6/16/2022 Delivered 6/26/2022.
Dealership - Stephen's Auto Center.
DRWright, 14U

Rank 0

Oct 21, 2020

#2
Good advice!
Hobgoblin

Rank 0

Oct 26, 2020

#3
I know there probably is an easier way to figure it out but I've always used the "manual" way of checking.
If you jack up the rear end so both tires are off the ground, you can look at the ratio of how many turns your driveshaft turns compared to 1 full rotation of the tire.
Place your truck in neutral and slowly rotate your tires while watching the driveshaft. Probably a good idea to mark the driveshaft with a clear chalk mark.
If you rotate your tires 1 full turn and your driveshaft rotates just past 4 turns, then you have 4.10 gears.
Sorry, I don't know what the popular rear end ratios are for a '71 plus they might not be stock anyways.

Hope this helps.
Deano, Found a formula for calculating diff. Ratio. Using my known #s- multiply RPM X TIRE SIZE, 3000 x 25"= 75000. Then multiply SPEED X 336, 55 x 336=18,480. Then divide 75000 ÷ 18,480= 4.06. I'm sure it would equal 4.11 if I figured exact RPM , just like you said!
LoneRanger, Deano Bronc
Moderator

RedNexican X Apalacian American: Goofyness!

Oct 30, 2020

#4
Or, since its a 9", dump the third member and start counting teeth! :LOL:

L.R.
AcesandEights, Deano Bronc

Rank VI

Oct 30, 2020

#5
Are you sure you have 25" tires? That's smaller than most cars. The tires on my bicycle are taller than that.
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matwats

Rank V

May 06, 2021

#6
Are you sure you have 25" tires? That's smaller than most cars. The tires on my bicycle are taller than that.
lol yeah that’s true you have point there.
"The core of mans' spirit comes from new experiences."

Rank 0

May 06, 2021

#7
lol yeah that’s true you have point there.
Yes, tires are 25", little shorter than stock. It's a cruiser, not a bruiser.

Rank 0

May 06, 2021

#8
I also want to have a more Highway friendly rear end ratio. Would it be cheaper to just put a whole used rear end in then actually changing the gears themselves?

Rank VI

May 06, 2021

#9
I have a 1971 302 4 speed, 25" tires. Absolutely no tag/ markings on Ford 9". Trying to figure out what differential gearing I have. In 2 Hi, at 55 mph I'm turning 3k rpm. Any guesses?
You try a marti report? Get the basic one, maybe its the stock rear? My '69 has 4:11. And it turns about that RPM maybe a tad high. Unfortunately I'm currently doing a new wiring harness so I can't take it out and tell you.
Btrain

Rank VI

May 06, 2021

#10
I also want to have a more Highway friendly rear end ratio. Would it be cheaper to just put a whole used rear end in then actually changing the gears themselves?

You probably need to change the front and rear. The Ford 9" is easy, but I don't know what's up front. Usually it's easier to change the gearing than the housing...well, that's not exactly true, but finding the housing you need with the spring perches where you need them (a vehicle with the same parts, but different ratio) is what makes it hard. Finding the housing with the gearing you want will probably be impossible. Changing the gears is a couple thousand dollars (front and rear), but easy for a qualified drivetrain mechanic.
2022 Jeep JLR, manual with even more extra guacamole
Sold - 2021 Jeep JLR, manual w/extra guacamole

Rank 0

May 06, 2021

#11
lol yeah that’s true you have point there.
You probably need to change the front and rear. The Ford 9" is easy, but I don't know what's up front. Usually it's easier to change the gearing than the housing...well, that's not exactly true, but finding the housing you need with the spring perches where you need them (a vehicle with the same parts, but different ratio) is what makes it hard. Finding the housing with the gearing you want will probably be impossible. Changing the gears is a couple thousand dollars (front and rear), but easy for a qualified drivetrain mechanic.
You probably need to change the front and rear. The Ford 9" is easy, but I don't know what's up front. Usually it's easier to change the gearing than the housing...well, that's not exactly true, but finding the housing you need with the spring perches where you need them (a vehicle with the same parts, but different ratio) is what makes it hard. Finding the housing with the gearing you want will probably be impossible. Changing the gears is a couple thousand dollars (front and rear), but easy for a qualified drivetrain mechanic.
I removed front shaft, no 4wd for me. Strictly an open roadster cruiser.
matwats

Rank VI

May 06, 2021

#12
As long as there is no front driveshaft and you never hook it back up, then you can buy a rear third member. I'd call Randy's Ring and Pinion and ask them. They'll need to know how many splines the axle shafts are and they can probably sell you a third member, which is easy-as-pie to change. Maybe $500 - $750???
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Sold - 2021 Jeep JLR, manual w/extra guacamole
Btrain

Rank VI

May 06, 2021

#13
@Btrain Another thing to which you have access is the VIN. The VIN, or VIN tag (the thing everyone wants to remove from the door frame) will often have an axle code, so you'll know exactly what the ratio is and whether it came with a limited slip, etc. The VIN will likely decode to show that, or if that tag is there and there is an "Axle" code, that too will decode to a specific ratio and open/limited slip/locker.
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Sold - 2021 Jeep JLR, manual w/extra guacamole

Rank V

May 08, 2021

#14
Hi could tell me what type of trans you have , I had the same problem, I was running a 4 speed NP, I change it to a zf5 with same rear and front axle, now the bronco run great with low rpm and great mpg, this will be chipper then changing the axle, some thing to think about , let me know if you have any questions
Bronco 70

Rank 0

May 09, 2021

#15
Can't really say. Engine is a '79 302, so I'm assuming 4 speed tranny is the same year. Did a manual test on differential and it came out to a 4.11.

Rank V

May 09, 2021

#16
Ok so if is a 4 speed np new process transmition that is what I had is a good transmtion for crowling rocks but if you want a more all around bronco check the ZF it will be like night and day good luck
Btrain

Rank II

May 24, 2021

#17
You try a marti report? Get the basic one, maybe its the stock rear? My '69 has 4:11. And it turns about that RPM maybe a tad high. Unfortunately I'm currently doing a new wiring harness so I can't take it out and tell you.
marti's not gunna help. It 50 yrs old w/ 1/2 that many PO (who could change tag, change gear, all sorta stuff). Count rotations. Go upa few sizes in tire for what U wanna achieve~
Chad - '70 LUEB on '77 frame (i.e. PS, D44, trapezoidal BB 9", 4.11) '69 4.1, NV 3550 & DSII, "T" D20 [email protected]~
Btrain

Rank II

May 24, 2021

#18
As long as there is no front driveshaft and you never hook it back up, then you can buy a rear third member. I'd call Randy's Ring and Pinion and ask them. They'll need to know how many splines the axle shafts are and they can probably sell you a third member, which is easy-as-pie to change. Maybe $500 - $750???
R&P is , oh wait he can't change 'em. Would B cheeper. Yeah, the whole center section'n a mechanic. 2 things pretty difficult fora 'new guy' - carb tune'n rear gear set up. They hafta B too accurate for a 'new guy'.
Hey ! congrats on the 2WD tho. Keep em runnin !!!
One guy did that and also lowered the rig. Looks pretty kouwell ? I kinda wish he chopped the top too. I guy I hate (foos) made a real sporty one that way~
Chad - '70 LUEB on '77 frame (i.e. PS, D44, trapezoidal BB 9", 4.11) '69 4.1, NV 3550 & DSII, "T" D20 [email protected]~

Rank V

May 25, 2021

#19
You probably need to change the front and rear. The Ford 9" is easy, but I don't know what's up front. Usually it's easier to change the gearing than the housing...well, that's not exactly true, but finding the housing you need with the spring perches where you need them (a vehicle with the same parts, but different ratio) is what makes it hard. Finding the housing with the gearing you want will probably be impossible. Changing the gears is a couple thousand dollars (front and rear), but easy for a qualified drivetrain mechanic.
Could you tell me what type of trans you have , I had the same problem, I was running a 4 speed NP, I change it to a zf5 with same rear and front axle, now the bronco run great with low rpm and great mpg, this will be chipper then changing the axle, some thing to think about , let me know if you have any questions
Bronco 70

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