How to figure out gear ratios without opening the box?

[Cornpanzer]

After reading all the posts on short gears, curiosity is getting the best of me. I there a plausable method for calculating what gears are in my car without opening up the box?

I know that I have some odd short gears in the box, but not sure which ones. TIA

[tfiv]

You could go to this gearbox calculator: http://www.geocities.com/z_design_st...n_993_bit.html, Put in the stock ratios, final drive, and your tire size, then drive the car and compare the RPMs you are turning in each gear at certain speeds, with what the calculator shows as stock. It would at least give you some idea how close or far you are from stock. If you have better math skills than I do, you may be able to work backwards and figure out exactly what's in there.

[Cornpanzer]

True, but with the short tires that I run, I cant accurately gauge my speed. Probably would get me close I guess.

You think its realistic to mark the crank pully and tire and turn the engine over by hand while measuring the rotation of the tire in each gear?

[911pcars]

After reading all the posts on short gears, curiosity is getting the best of me. I there a plausable method for calculating what gears are in my car without opening up the box?

I know that I have some odd short gears in the box, but not sure which ones. TIA"

Tom's link is one way to tell. You can also look at the gearbox number stamped on the underside of the gearbox housing, e.g. 901/02, 902/0, 915/xx, etc. If it hasn't been modified, look up the factory specs for the gearbox you have.

If there are no markings or the gearsets have been changed, use this method:

The overall gear reduction in each gear is the product of the gear ratio x the differential axle ratio.

If:
A = gear ratio
B = differential gear ratio,
then:
AxB = overall gear reduction

For example:
A = .96:1 (e.g. 5th gear)
B = 4.43:1 (typical 7:31 ring and pinion)

then .96 x 4.43 = 4.25:1 overall gear reduction

Thus, in gear position A, the engine rotates 4.25 revolutions for every 1 revolution of the drive wheel.

Working backwards, you can roll the car forward in each gear and count the number of turns the crank pulley rotates for every one revolution of tire rotation. Do this for every gear.

For example, chalk one of the rear tires at 6 o'clock, then roll the car forward in 1st one revolution (remove the spark plugs to make this task a little easier). If one revolution of the tire results in approx. 13.7 turns of the crank pulley. The overall gear reduction in first is then 13.7:1. Repeat and record the results for each forward gear. Create a Word table to keep track of your results.

To determine what part of that gear reduction is due to the gearbox ratio versus the differential ratio, divide each of the above by one of two commonly-supplied rear axle ratios.

Most 901 and early 915 gearboxes were equipped with a standard ring and pinion ratio of 7:31 or 4.42:1 (driven/drive teeth = gear ratio). The later 915 gearboxes have a 8:31 or 3.875:1. However, the factory provided some other ratios as well, but they're kind of rare. For example, I once had a 901-5-speed with a 6:29 differential (4.83:1), and some early 901-4-speeds had a 3.85:1 differential ratio.

Thus, either:
13.7 / 4.42 = 3.09 gear ratio or
13.7 / 3.875 = 3.53 gear ratio

In the 901 gear ratio chart I'm looking at, there isn't a 1st gear ratio that's as low as 3.53:1. So chances are the gearbox doesn't have a 3.875 differential. However, 3.09:1 equates to the gear designated as "A" (11:34 ratio). If it goes as smooth as this, assume the 4.42:1 gear ratio in all your calcs when repeating this for each of the remaining forward gears.

Something to do during the holidays?

Hope this helps,
Sherwood

[Cornpanzer]

Wow, thanks Sherwood!

Math brains amaze me. I was a hard working student - typically would have been close to straight A's in school, but I had to kill myself to get C's in math. Programing problem I guess.

Can I get the same result by jacking the car up on one side and spinning the wheel or will the diff slip enough to throw off the calculation. FWIW, I have a ZF limited slip.
Thanks

[911pcars]

My math brain is with you. I taught this stuff once upon a time so repetition has it's value.

With a limited slip, you can try to rotate one wheel in the air. As per my suggestion, pull the plugs so you're not fighting engine compression. Rolling the car on a flat surface might be easier though.

Sherwood

[Cornpanzer]

My 1st gear took 18.75 revolutions of the engine for one of the wheel. Assuming I have the 7.31 R/P, this results in a 4.24 ratio. (yes, I did it twice) I cant find a first to match.

My second gear is 12.25 revs of the engine for one of the wheel - .appears to put me in the range of a "B" second.........what am I doing wrong here?

[Cornpanzer]

Ok, Im a jackass!
When doing this, you must put your mark on the crank pully, not the fan pully.....LOL. Slow down and think Dave

My gears are as follow (Goofy Gears) A,E,L,R,Z. I guess I know now why it lugs down in fifth so bad! Can anyone tell me what should be stock on a 70S?

[Eade Hopkinson]

Ok, Im a jackass!
When doing this, you must put your mark on the crank pully, not the fan pully.....LOL. Slow down and think Dave

My gears are as follow (Goofy Gears) A,E,L,R,Z. I guess I know now why it lugs down in fifth so bad! Can anyone tell me what should be stock on a 70S?

If you have E as a 2nd gear, then you have a 904 shaft However, F is a stock main shaft, so are you sure E is second, or is F the second gear.

[Cornpanzer]

Eade,
I am pretty confident that my calculations are correct, but I will have to double check tomorrow. The difference between the E and F gears would be at least a half a revolution on the crank pully, so I think I have it right.
That would be a nice surprise eh?