How to determine gear ratio

[Gary Kemp]

My 63 327/340, 4 spd does not have the orignal differential and I would like to know what the gear ratio is. I currently have the trans out and the rear wheels off the ground. The diff is a non-posi unit. I marked a wheel, turned it once keeping the other wheel fixed and counted the turns of the diff front yoke. Result was the yoke turned approx. 2.1! times. Could this be correct? If so what ratio does this indicate. The engine in the car when I got it was a modded 396. I really don't know the correct way to determine this gearing. Help, anyone?

Thanx,
Gary

[Gary S.]

Re: How to determine gear ratio

Gary, if you will do a search of the archives you will see that this has been addressed before.

In brief, jack up the rear of your car, take it out of gear, parking brake off. Put a piece of masking tape at the 12 o'clock position on one tire and another piece of tape on the wheel well, just above the tire. Put a piece of tape on your DRIVESHAFT. Have an observor turn the wheel slowly while you count the turns that the driveshaft makes. 3 turns plus a bit more = 3.08; 3 1/3 turns = 3.36, etc. Takes all of five minutes and is very accurate, if you carefully position the tape.
Gary

[Stephen P.]

Re: How to determine gear ratio

Gary, The advice to research the archives is always a good one. I've found that most general question have been asked before. I think I remember a past post on this and that you will find that you need to double the number that you obtained since you don't have a positraction rear and only one wheel was turning. So it's probably a 4.11 rear.

[Duke W.]

Re: How to determine gear ratio

When you hold one wheel fixed you have to multiply the number of turns by two, so it's probably a 4.11, however, this ratio was only available with positraction from the plant. Only the base 3.70 ratio was available without positraction on SHP/FI engines. All optional ratios required positration, but if the clutches are completely worn and have no preload left, it will act as an open differential.

If you look on the bottom of the carrier for the coding you can probably determine if the carrier is original to your car and what ratio it originally carried.

On positration cars, when you turn the axle both wheels will turn assuming the clutches have any preload left, so the turns ratio you count is the actual axle ratio. With open differentials, sometimes both wheels will turn and sometimes not, so a good way to determine the axle ratio on a open diff. is to tie one wheel down as you did, count turns, then multiply by two.

Duke

[Gary Kemp]

Re: How to determine gear ratio

Duke:
Thanx for the response. The carrier code is AS with a date of 3 11 65. This would make it a '65 3.70 non-posi according to N.Adams Rest.Guide. I remeasured more carefully and beleive that it still is a 3.70.

Regards,
Gary

[Brian Monticello]

Re: How to determine gear ratio

One more thing.....since you're multiplying by two, any error in your measurements will be multiplied by two as well. Be sure to measure as accurately as possible.

You can also try to have an assistant rotate the other wheel so both are moving at the same time and you can take a direct reading - I've never attempted this and am guessing it might be difficult.