Ray. lift the rearend and sit it on stands.. put the car in neutral. mark a spot on the right tire and on the driveline.. Now rotate the tire one full revolution while counting the times the drivline turns. Figure any amount of turn after the origional mark on the driveline by way of minutes on a clock. This will give you the ratio of your rearend. As for the parts to switch to a posi.. Contact BGH at gears@muhlon.com for the desired ratio and he can get the parts to you at a reasonable cost. To upgrade mine from an open to a posi with complete new internals and 30 spline axels was in the neighborhood of 750 for the parts. Your local shop will charge you accordingly for the install.

 

I crunched some numbers in an RPM/speed calculator and even with 4.88 gears, at 55 MPH, the RPM would be around 3500. You either have very short gears or the tach/speedometer is off.

A more accurate way to determine gear ratio would be to lift one tire off the ground. Count how many times the driveshaft turns for two turns of the wheel. The reason I say lift one tire off is because you say your rearend isn't a posi. If you lift both tires off the ground, both tires might not rotate together and will throw off the reading. And the reason for using two tire rotations is to compensate for the differential turning twice as fast when one tire is on the ground.

Larry

 

Thanks Guys,

I will give it try. It does seem to rev up pretty high very quickly, needless to say I can go through a half tank of gas in about 60 miles. I will look into the tach though, It is a Tic Toc Tach and I am still using points. This may be a dumb question, but can you give me an example of the calculation, say I turn tire around twice and get 4 revolutions of the drive shaft (plug in more correct numbers if I'm way off) what would be the ratio calculation i would use and what would the final gear ratio be?

Thanks
Ray

 

I should have kept going with the procedure! If the driveshaft turns 4 times, then you would have a 4:1 gear ratio. When the final turn goes only part way around, that's when you would have to estimate the decimal of the ratio. For example, a 4.56 ratio would turn about four and a half times. A 3.73 would turn about three and three quarters. This procedure is only an estimate, but since the available gear ratios are somewhat spread out, you'll be right on the money 95% of the time. Here's some ratios that I looked up that's available for a 12 bolt GM rearend: 3.08, 3.42, 3.55, 3.73, 3.90, 4.10, 4.56, 4.88, 5.14, 5.38, 5.57, 5.86, 6.14.

Here's the calculator I found:
http://www.car-videos.com/tools/speedrpm.asp

I used a 225/70-15 for the tire size. I think that is close enough. If you a very short tire, the RPM will go up for a given road speed. You might have a 6.14 gear according to th calculator! That is steep!

 

These are all approximates on one turn of the tire to ___ turns on the driveshaft

Driveshat revolutions...
3&1/2 times = 3.55's
3&3/4 times = 3.73's
4&1/8 times =4.11's
4&1/2 times = 4.56's
4&3/4 times = 4.88's
5&1/8 times = 5.13's

If you turn the tire two times then you'll divide the number by two to get the correct number. If you turn the tire two times and get 4 turns on the driveshaft, then you'll have a 2.?? gear. Get your count and repost ...ok?

 

Thanks Guys,

I really appreciate it, I will get it on the floor jack today and see what I come up with.

Thanks
Ray

 

Ray,

Why don't you just pull the cover and look and see what it is??

That way, you will know for sure!

Heck, it will take longer putting the grease back in it then anything else....................

pdq67

 

There is also a 3.36 ratio for 12 bolts, they just don't make them anymore. I sold a 12 bolt with 3.36 gears not too long ago. It is a sought after ratio.

With your rpm at highway speeds there is no way you have 3.36. I would guess 4.56-4.88 depending on tire size, could be 4.11's. This is all assuming you have 1:1 4th gear ratio. If you post the tire size that will help figure it out.

One last thing you need to know it your speedo is accurate, since the rear gear has been changed, there is a good chance the speedo gear in the tranny is WAY off. This will make your speedometer way off (low or high it depends). Have a car pace you and this will be more accurate.

 

Well , things are wierd,

My tire size is P245/P60515 BF Goodrich T/A. I turned the tire two revolutions
and the drive shaft turned 3.3 times. I tried it twice to make sure. According to the specs I should have around a 3.55 or lower but that does not match up with the tach or speed. I think I'll have someone pace me to check speed. Any thoughts?

Thanks
Ray

 

Ok, something very off. You turned the tire 2 rev's and got 3.3 (3 3/4's ?) and was this after dividing by 2? Did both tires turn same way? Were both off the ground? Is it a open or posi ?

I have 3.73's and was running a 245/60/15 (26 1/2" tall tire) and at 65ish was about 3300 rpm with an automatic in drive (1:1). Maybe your tach is off ?

 

Hi Doug.

I turned the tire twice, and the revolution of the drive shaft was a little more than 3 and a forth turns. I rounded to 3 and a third turns. I did not divide by two. These were the raw numbers and one tire was on the ground. It is an open rear end.

 

Get someone to pace you. Somethings way out somewhere, Just can't think right now.

 

I though 10 bolts got 3.36 gears and 12 bolt's got 3.31 gears????

AND they aren't re-popping the 3.31 gears yet........

Wished they were, b/c, imho, they are just about ideal to me!!

You can turn a tire, but yet still run long distances down the road..

pdq67

 

Another Dumb question, I am assuming this is an open rear end as I CAN turn one wheel without the other turning. Would this be a corrrect assumption. I seem to remember when I had both rear wheels jacked up off the ground I can turn one wheel and the other wheel will rotate the other way. This has to be an open, right.

Thanks
Ray

 

Another Dumb question, I am assuming this is an open rear end as I CAN turn one wheel without the other turning. Would this be a corrrect assumption. I seem to remember when I had both rear wheels jacked up off the ground I can turn one wheel and the other wheel will rotate the other way. This has to be an open, right.

Thanks
Ray

Yes. It sounds like you've got 3.31 gears.

 

You are correct. If the driveshaft remains stationary, the other wheel will turn opposite the direction of the one being turned. Actually a posi (limited slip) will do the same thing, only it will be a lot harder to do so, depending on the condition of the clutches. All differentials behave this way. That's why I suggested keeping one wheel on the ground to eliminate any unequal wheel rotation.

 

Thanks 69-er

I am still finding things that are confusing. A friend told me the quickest way to check for posi is to take it out and let her rip and see if you lay a single or a double line. I did that tonight and let it smoke for about 20 feet before I backed off ( new tires, save tread). I had a perfect double line for the entire 20 feet. Does that mean posi for sure? And if it is ,what does that do to my gear ratio check of 3 and a third driveshaft turns for two full wheel revolutions?

Thanks
Ray

 

This 3 1/3rd thing has me puzzled. Gears are ...say 3.55:1 ratio. This means the drive shaft would turn 3.55 times to 1 tire revolution. So your 3 1/3rd to 2 revs is looking like a 1.something? and isn't possible.

 

Hi Doug,

If I pull the cover off and look at the gears, What am I looking for to verify the gear ratio. When I did the check, I had the transmission in neutral, ( 4 speed) , One wheel jacked up off the ground, I put a piece of tape on the tire and a piece on the drive shaft, I started with the tape at the top of the tire and rotated the tire two times, I watched the drive shaft and the tape on the drive shaft passed me three times and one third. Did I miss anything? I'm know I'm missing something obvious because I agree it does not add up to the calculations. From my previous double line test does this change the mix?

Thanks
Ray

 

Hey Doug, read my post at the beginning of this topic to see why he needs to place one tire on the ground. I think it will clear things up.

Ray, you are doing the check correctly. 3-1/3 turns should be a 3.36 or 3.31, whatever is correct for a 12 bolt. As far as the double line burnout, a car typicaly spins the right tire because of the torque reaction that lifts the that tire, causing it to loose traction. Your burnout result does not necesarily mean you have a posi. When you are doing your gear check, if the tire you are turning turns easily with the other on the ground, you either have an open rear end or a worn out posi. A worn out posi would display the same characteristics of an open rearend.

I just thought of another possiblity. Were factory tachs available with 6 cylinder engines? If so, if your tach was calibrated for a 6 cylinder, then the extra two pulses per revolution of the engine would cause it to read high.

If you are going to pull the cover, (the best way to resolve this), there are two ways to go about it. Count the teeth of the ring and pinion and divide the ring teeth by the pinion teeth. That will be your ratio. Or, on the ring gear, you will find a series of numbers that designate the number of teeth on each gear. It's easier to show you than describe which numbers are which, but look for numbers in this range: 11, 12, 13, 14 and 40, 41, 42, 43 give or take a couple. I think they will be separated by a colon : Divide the two like you would with the tooth count to get your ratio.

Larry

 

Were factory tachs available with 6 cylinder engines?

I checked my White Book, only V8's with consoles had tachs. Worth a shot...

Larry

 

This 3 1/3rd thing has me puzzled. Gears are ...say 3.55:1 ratio. This means the drive shaft would turn 3.55 times to 1 tire revolution. So your 3 1/3rd to 2 revs is looking like a 1.something? and isn't possible.

Maybe he tried it twice. 3 1/3 each time.

 

Check the code on the passenger side front of the axle tube and give us the number. You may need to use a wire brush to scrape the crud off first. This code will give you your ratio and whether or not it was a factory posi. Again, your findings are indicating a 3.31 rear, not 3.55. (3.55 would be just over 3 & 1/2 turns, get it?). Try the driveshaft revolution test with both wheels off the ground and you should be getting 3 & 1/3 to 1 ratio(or 3.31).

 

Hi Doug.

I turned the tire twice, and the revolution of the drive shaft was a little more than 3 and a forth turns. I rounded to 3 and a third turns. I did not divide by two. These were the raw numbers and one tire was on the ground. It is an open rear end.

I did ask that.


69-er , I did read and re-read your post and I agree about the one wheel on the ground. But I can't understand this 3.3 turns to 2 rev's unless Ray blinked,lol.
Also I only used 3.55's for an example, I have no clue why.

 

I will try in the next day or so,( I have to work late tomorrow night) to.

1. Find the numbers on the front of the axle
2. Either try the try the two wheels off the ground test, or pull the rear cover off and check if I cannot find the numbers on the axle.

I appreciate everyone hanging in there on this problem with me. Not that I like problems but when one comes up it's an obsession with me to find the answer. It's like a challange , with help like this on this forum it also is kinda fun. I guess misery loves company.LOL


Thanks All,

Ray