OK, I promised this some time back, but have just got around to it. Sorry.
To find your early Holden differential ratio:The easiest way is to get under your car and look at the flange/dust-shield on the nose of the diff.
If it’s a genuine “grey” diff, there will be no numbers on it at all, as they were all 3.89:1, and didn’t need numbers to distinguish them from other ratios.
If there is a number, it will probably look like this,
and you will know the diff is from a later (non-grey) model.
If the diff is out of the housing, you will find the ratio stamped on the crown wheel.
To verify the ratio when the diff is still in the car:Firstly, park the car on a flat level surface.
Make a chalk mark on the flange and diff housing as shown below.
Also, mark the tyre and floor.
Next, roll the car forward until the tailshaft has completed one full revolution and the diff marks line up precisely.
Mark the floor again.
Next, roll the car further forward until the tyre mark is again on the floor.
Mark the floor.
Measure the distance from the first mark to the second mark. (1 tailshaft revolution)
Then, measure the distance from the first mark to the third mark. (tyre circumference)
Divide the tyre revolution (in my case 1775 mm)
by tailshaft revolution (in my case 500 mm)
and hey presto…. diff ratio. (3.55)
Now the interesting bit…
500 mm in 1 revolution = .5 km in 1000 revolutions.
Therefore, at 1000 rpm, I will travel .5 km in 1 minute.
Multiply by 60 (minutes per hour), and that equals 30 kmh per 1000 rpm.
Now if I had a tacho, I would know that about 3300 rpm gives me 100 kmh.
Or, if I used a gps to check my speed, I would know that 100 kmh equals about 3300 rpm.
Either way, I could note my speedo reading and file away in the grey matter for reference.
So all you have to do is measure the distance travelled in 1 rev of your tailshaft (in mm), and multiply it by .06 to give you kph per 1000 rpm. Is that simple or what?
The calculation above works with any diff and any tyre size providing top gear is 1:1,
eg no overdrive.
Cheers,
Another Gaz.
P.S. Let me know if I'm in error, or if there's a simpler procedure.
