Question on rear axle gear ratio

[Muellers69GMC]

I have a Dana 60 rear diff. and I cannot find the BOM anywhere, nor the date of manufacture for that matter. I am trying to verify the gear ratio and the only thing I can find is a metal tab on one of the diff cover bolts that says:

41-10
4.10

I can only assume this means that it has a 4.10 ratio, but I would like to be sure. The truck is a '69 3/4T GMC with the 350/TH400 combo.

[mikajo39]

I'm pretty sure 41-10 means you've got 4.10's in the rear. Another way of checking is getting on the freeway and see if you can cruise at 65 mph without hitting 4000 rpm

[Muellers69GMC]

Thanks. I would if I could, but the engine and tranny are out getting rebuilt. This is actually the reason for the question. I am trying to make an appropriate cam selection for the 383 I am building. I want to have power in the appropriate range for the gearing.

I don't (or didn't) have a tach in the truck, but it did seem to be wound up good at highway speeds. When I let off the gas it did slow down quickly. Hard to imagine since this truck spent most of its life with a camper in the bed traveling down the highway all around the country.

Originally Posted by mikajo39

I'm pretty sure 41-10 means you've got 4.10's in the rear. Another way of checking is getting on the freeway and see if you can cruise at 65 mph without hitting 4000 rpm

[mikajo39]

Hmm, I would think if its a camper special it'd probably have 4.10's to help it give it that extra grunt and actually 4.10's wouldn't be to bad for a little more racey cam. It'll help it build up from idle into it's power range quicker. If you are really worried you could build the engine how you want it and change the rear gears later on. Since your building a 383 you should have some more torque than your average 350 .

[Longhorn Man]

a camper special is a ford, or a later model GM...
A custom camper has nothing to do with gear ratios though, that option package is nothing but HD springs, shocks, tires, badging and an anti roll bar.
the standard ratio in a 3/4 ton with a small block and an automatic is 4.10 gears.
At 60 mph, the RPMs would be closer to 3000 RPM... not 4000.
You can remove the cover and do a tooth count, that is the best way to find out whats in there, and if you just put the cover back on and roll with it, then you'll know the oil isn't 30 years old.
I would ditch the Dana if that 383 is going to make much power, the axle shafts aren't all that strong.

[WorkinLonghorn]

My "Dana" had a similar tag. Yes that is your tooth count and ratio.

[andy applegate]

another quick check
1- jack up both wheels
2- place a chalk mark on the drive shaft and tire for reference
3- rotate the drive shaft 1 complete turn while watching how many times the tire rotates. (a little past 4 rotations would mean a 4.10) (a little less than 4 rotations would be a 3.73) (3 and 1/2 rotations would be a 3.50) ect. ect.

this check would be for reference and get you close with out pulling the backing plate and counting ring and pinion teeth Andy

[special-K]

It`s the ratio on that tag.That makes it easy.
Does your truck have leaf springs? My Custom Camper had the same rear and I ran it to the Grand Canyon North Rim from Maryland and back,It loved the hiway.Hauls the camper like it`s not there.

[DANTIP]

41, 10 means there are 41 teeth on the ring gear and 10 teeth on the pinion gear. 41/10 = 4.10 or 4.10:1 ratio.

Certainly someone could have changed gears at some point but it started life as a 4.10.

You can check by rotating the drive shaft (if installed), if it is a 4.10 it should take just over four revolutions of the drive shaft to rotate the driven wheel 1 full turn.

[Beelzeburb]

Or just take the diff cover off (you'll probably want to replace the fluid anyway if you don't know the history of the rear end) and count the teeth, or read the tooth number stamped into the ring gear. I had my D60 cover off last week, and clearly stamped into the ring gear was 49:10 as well as other numbers.

[special-K]

why go through anything if the i.D. tag is right there staring at you?It`s a blessing.They usually aren`t there anymore because they get tossed when the cover comes off.I`m sure if the gears were swapped-out the tab would be long gone.Most likely the case has never been opened.

[Muellers69GMC]

This is likely to be the case. I am pretty sure I can trust the tag and assume the gear ratio hasn't been changed, but I'll probably pull the cover just to change the oil and clean things up. While I am there I can be sure.

Yes the truck has rear leaf springs. Are you suggesting that this is why I can't find a BOM on the axle?

Originally Posted by special-K

why go through anything if the i.D. tag is right there staring at you?It`s a blessing.They usually aren`t there anymore because they get tossed when the cover comes off.I`m sure if the gears were swapped-out the tab would be long gone.Most likely the case has never been opened.

[lakeroadster]

Originally Posted by andy applegate

another quick check
1- jack up both wheels
2- place a chalk mark on the drive shaft and tire for reference
3- rotate the drive shaft 1 complete turn while watching how many times the tire rotates. (a little past 4 rotations would mean a 4.10) (a little less than 4 rotations would be a 3.73) (3 and 1/2 rotations would be a 3.50) ect. ect.

this check would be for reference and get you close with out pulling the backing plate and counting ring and pinion teeth Andy

The method for a posi -vs- an open differential is different....

To check rear end ratio jack up one tire if you have an open diff, or both tires if you have a working posi or locking differential. Rotate the tire one full revolution for posis and lockers and 2 full revolutions for open diffs. Carefully count the number of full revolutions the driveshaft makes. This is your gear ratio. In other words, if the drive shaft turns 3 ¾ turns, you probably have a 3.73 gear ratio. Turning the tire for twice the number of full revolutions and dividing the drive shaft revolutions by two will give you a more accurate reading.

[leddzepp]

Why would the method for posi and non posi be different? The gear ratio is a constant no matter which one you have.

[Longhorn Man]

an open diff and a posi work different, the above is acurate.

[leddzepp]

I have done it on both posi and non posi rear ends, and it comes out the same

My 72 SWB has posi, and it took just over three turns of the driveshaft to get the tires to make one full rotation, leading me to the conclusion it has 3.07 gears.

The chassis I am restoring is an open diff, and it took 3 3/4 turns of the driveshaft to get one tire to make one full rotation, leading me to the conclusion it has 3.73 gears.

When I did this, I had both tires off the ground in both cases...so should I do this on the non-posi diff with one tire on the ground? Is that what makes the difference?

[69TowRig]

If you jack up the rear axle and both wheels are off the ground the tires should turn in unison regardless of open/limited slip/locker. That is of course unless you have a brake dragging.

[leddzepp]

Originally Posted by 69TowRig

If you jack up the rear axle and both wheels are off the ground the tires should turn in unison regardless of open/limited slip/locker. That is of course unless you have a brake dragging.

Yes....but the question is about how to determine gear ratio.

[lakeroadster]

Originally Posted by leddzepp

When I did this, I had both tires off the ground in both cases...so should I do this on the non-posi diff with one tire on the ground? Is that what makes the difference?

Having both tires off the ground will work with the open differential. The reason I posted above is I just checked my '65 the other day while up on a lift. It has an open differential, the drive shaft turned 3-3/4 turns, which means I have a 3.73 gear ratio. This is confirmed by the rear axle code that is stamped on the rearend axle tube which is "HA" = 3.73:1 ratio.

The key to making this work is Rotate the tire one full revolution for posis and lockers and 2 full revolutions for open diffs.

[haysonj]

Originally Posted by lakeroadster

Having both tires off the ground will work with the open differential. The reason I posted above is I just checked my '65 the other day while up on a lift. It has an open differential, the drive shaft turned 3-3/4 turns, which means I have a 3.73 gear ratio. This is confirmed by the rear axle code that is stamped on the rearend axle tube which is "HA" = 3.73:1 ratio.

The key to making this work is Rotate the tire one full revolution for posis and lockers and 2 full revolutions for open diffs.

Can you tell if you have a posi or open by rotating the tires