Axle Ratio Mind Boggler

[Silver Burner]

So as it turns out, I discovered today that my Sterling rear axle is actually a 3.55 ratio and NOT the 4.10 that I thought it was! Since I'll be doing my 4X4 swap and my parts truck has 3.55 axles, I thought I was going to have to either swap rear gears or rear axles, but now I just have to put in the D50 w/ 3.54's in the front!
This all started when I was working on my truck and got under there and saw a "55" on the axle tag followed by the 10 25 and I went Holy Sheet. So I got online and plugged in some rpm numbers with my tire size and sure enough, based on my speeds at certain RPM's, I really do have a 3.55 rear end! I always figured that 55mph doing 2000 RPM's meant I had to have 4.10's, but doing the calculator shows I'd only be doing about 46 or 47 at 2000 if I had 4.10's.
So I guess a hell yeah is in order. Less work for me!!

 

[THECACKLER]

I'd pull the cover and change the lube and while you're there roll the ring gear and look for markings at the outer edge. Usually their stamped with the number of teeth on the pinion and ring which can be divided to get the ratio. If not you can count the teeth. Beats snapping a front axle. That usually ruins a lot more than the axle.

 

[k_williams1982]

So as it turns out, I discovered today that my Sterling rear axle is actually a 3.55 ratio and NOT the 4.10 that I thought it was! Since I'll be doing my 4X4 swap and my parts truck has 3.55 axles, I thought I was going to have to either swap rear gears or rear axles, but now I just have to put in the D50 w/ 3.54's in the front!
This all started when I was working on my truck and got under there and saw a "55" on the axle tag followed by the 10 25 and I went Holy Sheet. So I got online and plugged in some rpm numbers with my tire size and sure enough, based on my speeds at certain RPM's, I really do have a 3.55 rear end! I always figured that 55mph doing 2000 RPM's meant I had to have 4.10's, but doing the calculator shows I'd only be doing about 46 or 47 at 2000 if I had 4.10's.
So I guess a hell yeah is in order. Less work for me!!

I was turning around 2,400 rpms @ 55-60 mph (IIRC) with the 4.10 gears in my '85 with a C6 and 35" tires.

 

[sle2115]

Yeah, with stock height tires, 4.10's in my truck were 2650 or so at 60 MPH.

 

[Silver Burner]

I was turning around 2,400 rpms @ 55-60 mph (IIRC) with the 4.10 gears in my '85 with a C6 and 35" tires.

A C6 in 3rd runs a 1:1 ratio just like a T-19 in 4th, correct?

 

[argve]

Here is a sure fire way to tell what ratio you have... and requires no fluid or gaskets.

Jack rear end up.

Place tranny in neutral.

Place a mark on the drive shaft and tire.

Turn tire by hand and count how many times the drive shaft spins for one turn of tire.

This can't lie.

Tags on covers, door jams, or pieces of paper can.

If while rotating the tire of choice if you look over and see the other tire turning the same direction as the one you are rotating by hand you have a limited slip or locker rear end, if the other tire is spinning the other direction then you have an Open Diff or peg leg kicker as I call them.

 

[Silver Burner]

I know about the spinning axles and you are correct, it certainly doesn't lie. I'm nearly 100% positive my rear really is a 3.55. The calculation numbers don't lie either. I'll do the axle spin for the D50 though, just to make sure they match.

 

[65sixbanger]

You can also count the number of teeth on the ring gear, then divide that by the pinion teeth.

 

[plywood]

Here is a sure fire way to tell what ratio you have... and requires no fluid or gaskets.

Jack rear end up.

Place tranny in neutral.

Place a mark on the drive shaft and tire.

Turn tire by hand and count how many times the drive shaft spins for one turn of tire.

This can't lie.

Tags on covers, door jams, or pieces of paper can.

If while rotating the tire of choice if you look over and see the other tire turning the same direction as the one you are rotating by hand you have a limited slip or locker rear end, if the other tire is spinning the other direction then you have an Open Diff or peg leg kicker as I call them.

True, if both tires are spinning at the same rate, then exactly one turn of each will tell you the ratio by counting the number of driveline spins.

If the other tire is spinning the opposite direction at the same rate, the driveline won't be spinning at all.

I always recommend leaving one tire on the ground and spinning the jacked up tire two turns due to the gears in the middle of the ring gear and the 2:1 or 1:2 ratio they cause.

Think of it like this, if you have a 3:55 and spin the driveline 3.55 times, then both tires will spin once, but if you hold one tire, the other has to spin twice to account for the other not moving.

 

[zpd307]

i remember my 87 f150 gasser..... had a 351ho, , and a c6 with 3.55s.

 

[dyoung14]

So as it turns out, I discovered today that my Sterling rear axle is actually a 3.55 ratio and NOT the 4.10 that I thought it was! Since I'll be doing my 4X4 swap and my parts truck has 3.55 axles, I thought I was going to have to either swap rear gears or rear axles, but now I just have to put in the D50 w/ 3.54's in the front!
This all started when I was working on my truck and got under there and saw a "55" on the axle tag followed by the 10 25 and I went Holy Sheet. So I got online and plugged in some rpm numbers with my tire size and sure enough, based on my speeds at certain RPM's, I really do have a 3.55 rear end! I always figured that 55mph doing 2000 RPM's meant I had to have 4.10's, but doing the calculator shows I'd only be doing about 46 or 47 at 2000 if I had 4.10's.
So I guess a hell yeah is in order. Less work for me!!

55 is right around 2000-2100 rpm area with 3.55 gears or so my GPS says anyway, about 2600-70 i think 3000 is 80 IIRC havebt drove the truck in a while and 3500 is like 90 but thats as brave as i get for now

 

[snicklas]

I would use Trav's (argve) trick but I would turn the driveshaft with both wheels off the ground and up on stands. Then turn the driveshaft and count the # of turns of the driveshaft to = one revolution of the tire.....

 

[Brianedwardss]

A C6 in 3rd runs a 1:1 ratio just like a T-19 in 4th, correct?

Correct, but the C6 will not be a true 1:1, account of the conventional torque converter not fully locking. I calculated it out once using a gps on the highway and seeing my rev's.. it was about 3% at 60 mph IIRC. Calculated out to about 1.031:1. My tires were brand new at the time, so I just ran all the stock numbers in a formula and came up with that answer.

 

[Brianedwardss]

The way I check rear axle ratios on trucks is pretty simple. Put the trans in N, lift only one rear tire off the ground, spin the tire one revolution while watching the driveline. It will only spin the number of times that your ratio is, not double. The difference between 3.55 turns and 4.1 turns isn't much, so I usually do it a couple times to be sure. Start with one side of the yoke facing you, parallel to the ground, and make a mark with something on it. This makes it easier to keep track and be more accurate.

 

[Silver Burner]

I will definitely do that. For the front axle also