The Stovebolt Site Forums The Shop Area General Truck Talk Starting with questions

I have decided to go from rear to front, i thought was the easier starting for someone with no experience on this.

I worked on the rear brakes. I found the axle is not the original. Looking for the brake parts, the guys on the parts store told me it was a 3/4 ton chevy 68 axle, at least drums and shoes are. There are no numbers printed anywhere, besides the point that if so, they probably will be MExican serial numbers. Is there anything i can do to identify the axle?

How can i know what can of gear ratio i have now?

What the gear ratio means?

Welcome aboard. The numbers for the axle should be stamped on the front of the carrier. It can be either a 4.10 or 4.56 ratio. My sources do not show any differences for countries, so i would think that the numbers for the axle would be the same as US. The basic axle is very similar from '47-'72, with the main difference being the brakes and the gear ratios. An easy way to check the ratio if you can not find the numbers is to make a reference mark on the driveshaft, then turn it as some one watches the axle flange. then count how many turns it takes on the driveshaft to make 1 turn of the axle shaft. Aslo, I read your other post. If the truck is a 1/2 ton, the trans has been changed. The '54 was still torque tube drive (closed type). The tranny would have to be changed to work with an open driveshaft.

You can jack up one wheel to turn it as Longbox55 sez while in neutral, but leave the other one on the ground otherwise it will counter rotate and mess up the turn count.

as for gear ratio, its how many times the engine spins for 1 turn of the rear wheels. With the heavy duty 4 speed (commonly known as the sm420) with the low first gear, its around 7:1, then piped through the rear end which could be 4.5:1. So the first gear means the engine spins 7 revolutions to every one turn of the driveshaft. Then the drive shaft will turn 4.5 turns to every turn of the rear wheel. So you end up with 31.5 turns of the engine for every turn of the rear wheels!

So the engine spins fast, the wheels spin slow, and you have a mountain of power. On the highway you want more speed and the mommentum of the truck helps it keep going. So in 4th gear, the tranny ration is 1:1 or direct, and every turn of the engine is 1 turn of the drive shaft, and that means the engine turns once for every 4.5 turns of the drive shaft (assuming a final drive of 4.57). So that means the truck moves along much quicker, but with much less power.

You'll find that the 4.57 will let the truck go around 50 without pushing it too hard. If you want to go faster you can plug in a different rear gear set of 3.90 or 4.10 which will get you up to around 60. You can also put on really tall tires on the back of the truck and they'll help it move even faster.

My truck had 5.26 gears as it was a 1 ton. I found some 4.10 gears (rear end) and that was a big help to go quicker on the highway.

Ok you may get lucky with this. Look for a tag attached to the diff. cover by a bolt that holds the cover on. There are numbers on the housing somewhere.

The 'gear ratio' is what everyone has said: It is the number of times the engine turns to make the tires turn. With a 4.10:1 ratio, the engine has to turn 4.1 times to turn the rear tires one revolution.

Inside the rear differential there are two gears. A ring gear and a pinion gear.

The ring gear is attached to the axels by means of a 'carrier.' If you lie the ring gear on the ground it will have teeth on one side and be flat on the other.

The pinion gear looks like a christmas tree, if the green part were gears and the tree part was attached to the drive shaft, (which connects to the transmission, which connects to the engine)

If you open up the rear differential and look on the outside edge of the ring gear you will see some numbers stamped on it. Those numbers tell you where it was made, the serial number, and the number of teeth the ring and pinion gears have. If you divide one number into the other you will know the gear ratio. For example, you may see the numbers that are on the ring gear in my 12 bolt posi:

2 GM 3862325 11 41 9 69

The 2 GM probably means the plant it was manufactured it (Eaton I'm guessing), the 3862325 is the serial number.

Th 9 69 is probably the month and year of the production.

The number of teeth on the ring gear is 41 and the number on the pinion is 11.

If you divide 41 by 11 you get the 'gear ratio' or the number of times the engine has to turn the pinion to turn the ring gear once. In this case 3.73:1.

There are other ways to get the rear differential ratio. For example, raise both the tires off the ground and turn the drive shaft by hand and count the number of revolutions it takes to make the one tire go around 10 times, then move the decimal point. I use ten revolutions to make it an easier number, like 41 or 37 (4.1 or 3.7) I also mark the shaft and differential with chalk, and the rim and brake backing plate to make it easier.

Another way is to open the rear differential and count the teeth on the ring and pinion gears and divide.

Knowing the gear ratio doesn't tell you every thing you need to know, but it gets you a long way.

Once you know the gear ratio for the rear differential and the gear ratios for your transmission you can calculate 'final gear ratio' for each gear. Most 4-speed and 5-speed transmissions have a 1:1 ratio in 4th gear. So with a 3.73 'final gear ratio.' With .72:1 5th gear ratio, like many of the t-5s have, your final gear ratio would be 2.69 (3.73 x .72 = 2.69).

You also have to take into account tire height to figure out the RPM's it'll take to go a certain speed. And with those bits of info and the max RPMs for your engine you can figure out the max speed.

When you know the 'power band' of your engine(i.e. what RPM range it produces the most power in), you can work backwards and figure out the best rear differntial gear ratio to use for your transmission and engine combo.

Here's A (just one) formula for RPMs. There are also many 'caculators' on line you can just plug the numbers into:

RPM =

(336)x(Rear Diff)x(Tranny Ratio)x(MPH)
_____________________________________

(Tire Diameter in Inches)

FOR EXAMPLE:

Rear Diff: 4.11:1
4th Gear: 1:1
Final Ratio: 4.11:1
Tire Hight: 29"
Speed: 65 mph

(336)(4.11)(1)(65)
__________________
(29)

89,762.40
_________
29

3,095 RPMs

The 4-speed with 3.08 rear differential ratio and same tires would need 2,300 RPMs, roughly!

The same rear differential with a fifth gear transmission ratio of .72, which is common on the t-5, would need 2,475 RPMs at the same speed with the same tires!

A five speed with a 3.42 rear differential would only need 2,300 rpms to go 75 mph!!!!! That means faster, less gas, less noise and stress on the engine, plus a big chunk of the power band still available to pass or speed!

All this info has been great.

I'm currently working on the axles and brakes. I will put all together next weekend and then to get the ration

Thanks

Didn't see anyone mention that when you check for ratio with one wheel off the ground, You will have to devide your answer by 2 as with only one wheel turning,it will turn twice as fast thru the diff. Correct me someone if I am wrong.