richbopp

I used the calculator program that was posted out here a few weeks ago to see what rear I have in my 64. I'm doing 50MPH and the rpm's are just about 2500 in 4th. After loading all the data into the calculator, including tire size etc, it appears I have 3.90 rear. Just does not seem possible; but who knows. From historical data I have, appears rear was not changed since birth.

What tag or stamp can I look at on the rear, and where is it located, to determine what gears are inside? Or where else should I look. And is this a true means to tell if the stamping is reflective of the inside gears? Can different gear ratios use same style cases?

Anything would be helpful. Thanks

ratmotortom

Just crawl under the car and look at the flat surface at the bottom of the differential carrier. There will be a date code as well as the ratio code stamped there. Post it here and we'll tell you what's in it !!

SWCDuke

You don't need a calculator. Record revs as closely as possible at exactly 60 MPH.

If your speedometer gearing is correct for the installed axle ratio:

Axle ratio = revs@60/760.

Tire size is immaterial, but non-OE tire revs/mile will affect speedometer accuracy.

Based on 2500@50 linear proportioning yields 3000@60 and 3000/760 = 3.95, but did you just read the tach to the nearest hundred revs, or was it dead on?

If you read the tach to three places or at least the nearest 20 or 25 revs, you will probably be able to determine if it's a 3.70 or a 4.11.

If the above doesn't work, read the axle code on the bottom of the carrier, or do both. The axle may not necessarily have the same ratio as what it was built with by GM.

Duke

Chuck Gongloff

You could always try the old "manual method". Jack up the rear of the car.

Put the car in 4th gear.

Put a chalk mark at 6:00 o'clock on one of the tires. Put a chalk mark at 6:00 o'clock on the drive shaft.

Rotate the wheel one full turn, back to 6:00 o'clock. While doing this, count the number of times the driveshaft turns.

3 1/2 times? 3.55

A little less than 4? 3.70

A little more than 4? 4.11

4 1/2 times? 4.56

Chuck

knight37128

3.70

steemin

This is what I have done in the past.
Works great..
Does the car have to be in 4th?
I realize that on these 4 speeds 4th gear is 1:1
Scott

66since71

Youre right. In fact it should be in nuetral so you can actual turn the wheels.

Chuck Gongloff

Doh. I meant NEUTRAL. Brain cramp. Chuck

ratmotortom

I wondered if you had a magic method of turning the wheels in fourth gear !!

Ironcross

I have a way but it`s really not exactly magical. In fact some may not think it`s funny either. Ready? Ok start engine, put trans in 4, and let the clutch out. See, simple! I know your thinking so am I. Ah, but the catch is mine is a 4.56 gear.

With all these serious issues presented on this forum I thought it was time to laugh at someone like me.

NEVERL8

Assuming, of course, someone has not swapped out the gears. My differencial says 3:36 non posi. Dead nuts 40 on the speedo is dead nuts actual 36 (according to the radar timing sign I passed by). Someone dropped in a 3:70 but never changed the speedometer gear. I had a clue that something was amiss when I saw the non original rear end cover. Now I know why everyone keeps passing me on the highway.

steemin

I am new here I was trying to be nice.
Scott

richbopp

Phew.... took it out for ride and guess what?...... never took note of anything we've discussed here. Was having too much fun just driving the Vette.

Oh well, there's always tomorrow. I promise to do the tach/speedo thing again tomorrow.... sure I do

AZDoug

I do.
Turn ignition to on, then start,and then count really, really fast as all those chalk marks go around on the tires and driveshaft.

Doug

K2

Or you can do it mathematically with the formula:
(RPM x tire dia.) / (MPH x 336)= Ratio

Example: If your RPM is 2000 at 60 MPH and tire dia is 30" then
(2000 x 30)/(60 x 336)= 3.00 Ratio

Now you can plug in any desired RPM at a given MPH and come up with the ratio you need to achieve that.

Chuck Gongloff

If you take the spark plugs out, it reduces the CR somewhat, making the entire engine easier to turn over. Chuck

buns

richbopp- just wondering , what is the diameter of your tires ? I'd like to check the accuracy of my first computer ( from the seventies ). According to my Isky computer , if your tires are 26" , then you do indeed have 3.90s in the rear.

richbopp

Tires are 215/75 15's. Now let me see, diameter... hummm, is this the time when 10th grade algebra is finally used? Diameter is the distance between what two parts, places? No just kidding

I know these are not the original spec tires/sizes.

I need to measure the diameter and get back to you. I remeber that measurement of diameter was a consideration in determining the rpm/mph/gear ration thingy...... bigger wheel rotation effect etc.

Let me see if I can get a tape measure and determine the exact diameter.

Rich

Procrastination Racing

Don't measure the diameter, measure the circumference. You will never get the diameter right. Circumference is even hard, as you need "rolling" circumference. Mark the tire and the ground. Roll the tire on the car until the mark hits the ground again. Mark the ground. Measure the two marks on the ground.

Circumference / Pi = Diameter

http://www.angelfire.com/fl/procrastination/rear.html

richbopp

Oh man.... this is getting way too complcated. I'll measue the distance the tire travels in one rotation....OK? After that, I'll need a few beers and maybe a bottle of wine to convine myself it's all worth it. I'm just going to buy a 3.07 rear and have it installed. We do highway driving out here in the Utah mountains..... as in high speed stuff; no need for off the line speed.

My 64 is a driver..... I need to DRIVE it....