How to Calculate Gear Ratio wo Removing Covers

Disclaimer: Links on this page pointing to Amazon, eBay and other sites may include affiliate code. If you click them and make a purchase, we may earn a small commission.

HotRodPC

Administrator
Staff member
Admin
Joined
Aug 29, 2010
Posts
47,008
Reaction score
8,976
Location
OKC, OK
First Name
HotRod
Truck Year
85 K20 LWB
Truck Model
Silverado
Engine Size
454 - Turbo 400 - 3.73
No doubt. Keep your racing limited to the Yugo's and Geo's. Don't be taunting anything else. :rofl: Your poor transmission is working it's ass off. I'd be keeping your eyes open for some 4.10 axles. Many times you can find a set of used axles drum to drum for about $200-$250. Finding 4.10's in half ton axles is pretty tough, but 3.73's are out there in half ton.
Other option is to find some 3/4 ton 8 lug axles drum to drum with 4.10's and those are pretty common. I've seen those go for $250-$325 a pair. But then you'd need new wheels and probably tires too.
 

roadwolf

Full Access Member
Joined
Sep 6, 2011
Posts
134
Reaction score
12
Location
France
First Name
Rodolphe
Truck Year
1987
Truck Model
silverado 10
Engine Size
5.0
As my goal is to build a fall guy truck replica, i need the 8 lug to be acurate, the only thing that slows me is the shipping cost to France for so heavy things and also i wan"t to keep the truck light.
In fact i am also loocking in converting my 10 bolt to 8 lugs, i know it can be done for the front, but i have not found anything for the rear and i also would like a rear disc axle/conversion.

I know that's make lot of thing, and it's a headache between what i would like (a light ls1 powered awd squarebody) and what can be done mainly from France.

thanks for your help
 

tiredtiretodd

Full Access Member
Joined
Jul 9, 2014
Posts
170
Reaction score
20
Location
NORCAL
First Name
Todd
Truck Year
75
Truck Model
K 25
Engine Size
350
With some simple math, anyone can determine your gear ratio without removing axle covers. You will need to know a bit about fraction and decimal conversion, but I'll put up some info here to make that easy for you. Most of are aware of the basics without even thinking about it. Examples, being 1/4 is .25, 1/2 is .50 and 3/4 is .75.

On to the good stuff. First thing you want to do is figure out if you have a locker, spool, some Limited Slip Diffs (LSD) or locked axles as in Lincoln Locked or welded spider gears making the open diff a locked axle OR, if you have an open differential or Limited Slip Diff or Locking Diff that doesn't lock the axles until a predetermined set of events happens to make them lock up.

This is easy to do. FIRST chock the front wheels in both front of the tire and rear of tires so it doesn't roll since your E brake goes to the rear wheels. Jack up the rear end with both wheels off the ground. I usually use a floor jack in the center of the axle. With both rear wheels off the ground. Spin one wheel. If the other wheel spins in opposite directions, then you likely have an Open differential without a locker or LSD, but G80 Locking Diff does also spin in opposite directions, so you could still have locking differential. That doesn't matter for figuring gear ratio though. Bottom line here, you're trying to determine if the wheels spin in opposite directions, or Not. If you go to spin the one wheel, and it doesn't turn at all, then you'll need to put the transmission in Neutral, and this is likely going to mean you have a locker, good LSD unit or welded spider gears. After the trans in N, spin the one wheel, and it's likely to have both rear wheels spinning in the same direction.

It's important to know this, if the wheels spin in opposite direction or if they spin the same direction. If they spin the same direction, then it's even easier to determine the gear ratio. If they spin in opposite directions, then you have another step of multiplication to do. So lets do the easy one first, and assume the wheels spin in the same direction.


I use the valve stem on the tire as marker and start my wheel spin count at 12, 3, 6 or 9 o'clock position. Whichever position I have the pinion yoke or U joint either straight across or straight up and down. It just makes it easier for an accurate count. Then you're off. Make 1 full tire spin by hand and count how many times the driveshaft, or the pinion yoke turns. If the driveshaft spins:


Wheels Spin in the SAME direction.

A hair over 2 1/2 times or 2.5 times, then safe to say ratio is 2.56:1
A hair under 2 3/4 times or 2.75 times, then safe to say ratio is 2.73:1
A bit over 3 times, then safe to say ratio is 3.08 or 3.07:1
A hair over 3 1/2 or 3.5 times, = 3.54 or 3.55:1
A hair under 3 1/2 or 3.5 times, = 3.42:1
A hair under 3 3/4 or 3.75 times = 3.73:1
A bit over 4 times = 4.10 or 4.11:1
A hair over 4 1/2 or 4.5 times = 4.56 or 4.57:1
A bit under 5 times = 4.88:1
A bit over 5 times = 5.13:1

You should have this by now, so that's as far as I'm going and anything else is very uncommon anyway in light duty trucks and cars for sure.

Now, if the wheels spin in opposite direction the procedure is a bit different.
You must put 1 wheel back on the ground so it stays stationary and you can get an accurate count. This is because if the wheel is moving, that means the spider gear to the that other axle shaft is moving and you don't know exactly how much and it makes your count inaccurate. So with just one wheel in the air, transmission in Neutral so that the driveshaft is able to spin, then count the pinion yoke spins to 1 full tire revolution. Now here's the difference and the added step. Take the # of spins and multiply it by 2 and that is your gear ratio. The reason you have to multiply it by 2, is because the distance you are moving the wheel, the spider gears are spinning backwards on the other stationary axle shaft gear, cutting your pinion yoke turns in half. So if you pinion yoke spins:

Wheels Spin in the OPPOSITE direction.

A hair over 1 1/4 times or 1.25 times, x2 then safe to say ratio is 2.56:1
A bit over 1 1/3 times or 1.33 times, x2 then safe to say ratio is 2.73:1
A bit over 1 1/2 or 1.5 times, x2 the safe to say ratio is 3.08 or 3.07:1
A hair over 1 3/4 or 1.75 times, x2 = 3.54 or 3.55:1
A bit under 1 3/4 or 1.75 times, x2 = 3.42:1
A heavy bit under 2 times x2 times = 3.73:1
A bit over 2 times x2= 4.10 or 4.11:1
A hair over 2 1/4 or 2.25 times x2= 4.56 or 4.57:1
A bit under 2 1/2 or 2.5 times x2 = 4.88:1
A bit over 2 1/2 or 2.5 times x2 = 5.13:1

And that's how it's done. Now noone should need to remove their diff cover, unless you're just wanting to do a differential oil change, to calculate your gear ratio. Which, if you don't know the history of your truck, it's not a bad idea to change the gear oil, and identify which carrier you might have when the cover is off. Keep in mind, some units require special oils like synthetic, or synthetic blend, some require GL4, some GL5, and some require LSD additive, so make sure you get a good ID on your carrier type if it appears to be a Locker or LSD.
Thanks for the link! I will try this.
 

LateOnTheBrakes

5.3-Swapped 86 CUCV
Joined
Feb 2, 2020
Posts
463
Reaction score
123
Location
Virginia
First Name
Joshua
Truck Year
1986
Truck Model
CUCV
Engine Size
5.3
I am taking my covers off to change the fluid this weekend. With the gears or carriers be stamped with the ratio of do I need to count teeth? Anything special to know to identify the carrier or just take lots of pics?
 

HotRodPC

Administrator
Staff member
Admin
Joined
Aug 29, 2010
Posts
47,008
Reaction score
8,976
Location
OKC, OK
First Name
HotRod
Truck Year
85 K20 LWB
Truck Model
Silverado
Engine Size
454 - Turbo 400 - 3.73
I am taking my covers off to change the fluid this weekend. With the gears or carriers be stamped with the ratio of do I need to count teeth? Anything special to know to identify the carrier or just take lots of pics?
They will be marked on the ring gear. Just spin it until you see all the #'s. Take a pic and post it up and someone here will be able to translate it for you.
 

LateOnTheBrakes

5.3-Swapped 86 CUCV
Joined
Feb 2, 2020
Posts
463
Reaction score
123
Location
Virginia
First Name
Joshua
Truck Year
1986
Truck Model
CUCV
Engine Size
5.3
I see lots of numbers. None that sound like a ratio to me unless it's 3.22. 3 22 73 Dana 30060 A3D47 43-14.
 

Vbb199

B-rate Hillbilly Customs
Joined
Jan 12, 2018
Posts
8,871
Reaction score
14,885
Location
Salisbury NC
First Name
Vince
Truck Year
89, 79
Truck Model
89 Suburban R1500, 79 C10
Engine Size
350, 502
"43-14"
43 ÷ 14 = 3.071


3.07 is your gear ratio.


Typically just called 3.08
 

Ricko1966

Full Access Member
Joined
Apr 11, 2017
Posts
3,938
Reaction score
5,688
Location
kansas
First Name
Rick
Truck Year
1975
Truck Model
c20
Engine Size
350
Busy so i didn't read the replies.Quick and dirty.Tape a string with a nut tied to it for weight to the driveshaft.Mark a tire at 12oclock push the truck 1 revolution of the tire, then count your string wraps. 3.5 wraps equal 3.5 gear 2.73 wraps is a 2.73 etc. If you want to measure your driveshaft diameter and length of string wrapped you can get exact, but just eyeballing wraps is close enough for me.
 
Last edited:

Vbb199

B-rate Hillbilly Customs
Joined
Jan 12, 2018
Posts
8,871
Reaction score
14,885
Location
Salisbury NC
First Name
Vince
Truck Year
89, 79
Truck Model
89 Suburban R1500, 79 C10
Engine Size
350, 502
You can also look on your DANA axle (dana only) along the axle tubes, usually the RH tube if you're facing the diff cover, and you'll find a stamped BOM number.


That can be translated on Dana's website.


Sometimes you get lucky on dana axles and the mfg tag will be on the diff cover, which will outline options (trak-lok, open, etc and gear ratio)
 

HotRodPC

Administrator
Staff member
Admin
Joined
Aug 29, 2010
Posts
47,008
Reaction score
8,976
Location
OKC, OK
First Name
HotRod
Truck Year
85 K20 LWB
Truck Model
Silverado
Engine Size
454 - Turbo 400 - 3.73
"43-14"
43 ÷ 14 = 3.071


3.07 is your gear ratio.


Typically just called 3.08
Jep, What he said. Grandpa's station wagon gear.
 

Vbb199

B-rate Hillbilly Customs
Joined
Jan 12, 2018
Posts
8,871
Reaction score
14,885
Location
Salisbury NC
First Name
Vince
Truck Year
89, 79
Truck Model
89 Suburban R1500, 79 C10
Engine Size
350, 502
Jep, What he said. Grandpa's station wagon gear.


That is a high gear, I'm picturing a old 70s 500ci caddy or a 455 buick and small tires right now haha
 

LateOnTheBrakes

5.3-Swapped 86 CUCV
Joined
Feb 2, 2020
Posts
463
Reaction score
123
Location
Virginia
First Name
Joshua
Truck Year
1986
Truck Model
CUCV
Engine Size
5.3
Yeah. Oh well, it's getting replaced anyways. Does the ring gear indicate the story it was built as well? I notice a "73" stamped on both the front and rear ring gear.

You must be registered for see images attach
You must be registered for see images attach
 

LateOnTheBrakes

5.3-Swapped 86 CUCV
Joined
Feb 2, 2020
Posts
463
Reaction score
123
Location
Virginia
First Name
Joshua
Truck Year
1986
Truck Model
CUCV
Engine Size
5.3
I also found the stamps on my front axle tube that I thought would be the BOM. The numbers don't match the pattern with the Dana 60 tool I'm familiar with. The Dana expert tool websites also won't load for me. Does anyone know how to translate this?

You must be registered for see images attach
You must be registered for see images attach
 

Vbb199

B-rate Hillbilly Customs
Joined
Jan 12, 2018
Posts
8,871
Reaction score
14,885
Location
Salisbury NC
First Name
Vince
Truck Year
89, 79
Truck Model
89 Suburban R1500, 79 C10
Engine Size
350, 502
I also found the stamps on my front axle tube that I thought would be the BOM. The numbers don't match the pattern with the Dana 60 tool I'm familiar with. The Dana expert tool websites also won't load for me. Does anyone know how to translate this?

You must be registered for see images attach
You must be registered for see images attach


If it's older than a certain year, like 1980 or something, the dana website won't have data on it.
You gotta look thru their archives or something.

I found an. Old archive thru pirate 4x4 one time for a 60's model dana 60 I had.
 

LateOnTheBrakes

5.3-Swapped 86 CUCV
Joined
Feb 2, 2020
Posts
463
Reaction score
123
Location
Virginia
First Name
Joshua
Truck Year
1986
Truck Model
CUCV
Engine Size
5.3
No, I mean the Dana website for looking at stuff up comes up with an internet error. Like they decommissioned the website.
 

Forum statistics

Threads
41,852
Posts
903,520
Members
33,365
Latest member
rossivey
Top