How to Determine Your Transmission Final Drive Ratio - In the Car or on the Bench

Wobblybob
Elite Member
Joined: January 29th, 2009, 12:21 am

March 20th, 2011, 1:44 am #1

Here's the procedure for determining the final drive ratio of a transmission while it's still in the car.

See my post of April 7, 2011 (page 2 of this thread) for doing it to a transmission on the bench.


Also, pacapo deserves a lot of credit for this whole concept because it was his original post that put me onto the idea of turning the engine over so many turns and counting wheel revolutions. I neglected to mention him in my original post because I couldn't remember where I saw it in the first place. It was his post that started all this (and he never even mentioned it when he complimented me on the original post).

While getting my 94 XFI parts car ready for the tranny and camshaft swap I discovered some yellow junkyard paint on the valve cover. This, of course made me think it might not be the original motor. That made me start worrying that if the engine has been changed, maybe the transmission has too.

I pulled the valve cover and measured the lobes on the cam and found, much to my disappointment, that several measured 1.592" which means it isn't an XFI cam. (XFI's = 1.5602 to 1.5665, standard 3 cylinders = 1.5911 to 1.5974).

Now I was really worried about the transmission! How can I tell which one it is?

Although I found a few guides on the forum I still wasn't sure how to tell. So, using gear ratios and final drive ratios I calculated how many turns an elevated wheel would make per revolution of the engine in various gears. Although I was really only concerned about my transmission I figured I might as well calculate all four possible final drive ratios for the benefit of others. I finally selected the 5th gear numbers to use since they result in the greatest wheel angular difference per engine revolution.

I then performed the procedure outlined below on three different 5 speed Metros that I own (yeah three, ask my wife!): my '95 3 cylinder; the '94 XFI parts car; and my '93 3 cylinder base model.

And the results were just as I hoped and expected (and, hooray, I learned that I do have an XFI transmission!).

Here's the procedure to use:

- Block one or both back wheels
- Key off, put the car in 5th gear, pop the hood release
- Elevate either front wheel of the car (the right wheel is easiest - see below). The other front wheel must remain stationary.
- Open the hood and rotate the elevated tire until the timing mark on the crankshaft pulley is lined up with the indicator (pick either end, or whatever you can see best)
- Put a mark on the tire at BDC (bottom dead center) (it's OK if it's slightly off - don't go crazy getting it exact). Mark the ground too, for reference.
- Watching the crankshaft pulley, rotate the tire CLOCKWISE until the ENGINE has made exactly three revolutions
- Observe the location of the mark on the tire to determine which final drive ratio you have

4 o'clock = 4.39
5 o'clock = 4.10
7 o'clock = 3.79
9 o'clock = 3.52

Below are pictures of the 3 different cars and three final drive ratios. I actually performed the above procedure on all three cars and did not fudge the results. These are honest wheel locations after three engine revolutions. Because I wanted to have pictures of all 4 ratios, I simulated what the 3.52 would look like by turning the XFI wheel to the 9 o'clock position and taking a picture. I'm sure it's correct though.

I elevated the right front wheel on all the cars beacause I was working alone and you can see the crankshaft pulley while rotating the right wheel but you can't when rotating the left wheel. If you must use the left wheel remember to turn it clockwise. Also the position of the BDC mark will appear exactly as is does in the photos: 4 o'clock, 5 o'clock, etc, not reversed.

This is where you start.



Rotate the engine exactly 3 revolutions (turning the tire clockwise to rotate it) and look to see where the BDC mark ends up.

Below is a '95 three cylinder, 4.39 ratio



Below is a '93 three cylinder, 4.10 ratio



Below is a '94 XFI, 3.79 ratio



And lastly, here is what a 3.52 ratio would look like (I faked this picture because I don't have a 3.52 - it's accurate though)



If you need to check out a transmission and can't remember these numbers or photos remember this: the "good" ones (3.52 & 3.79) produce MORE than 2 wheel revolutions and the "bad" ones (4.10 & 4.39) produce LESS than two revolutions. And the extreme ratios (4.39 and 3.52) are both about 90 degrees away from two wheel revolutions.





Last edited by Wobblybob on April 30th, 2011, 2:04 am, edited 10 times in total.


"The Flying Egg" 3/5 and "Otto" 4/auto
Quote
Like
Share

Good bye
Elite Member
Joined: December 3rd, 2008, 4:48 am

March 20th, 2011, 1:53 am #2

Very nice write-up. Thanks, that is easier than counting the crown gear teeth if the car is all together.
I am metroless currently, just trolling the sight! This car has a new owner.
91 Metro, 93 1.3L 8-valve and 3.5 tranny, Mk3 harness with live data stream, late front brakes, Swift GT rear discs, late sway bars, Esteem seats, 175/65r 14's on GT wheels. Lsi headlights, Mk 3 Swift GT bumpers,44 mpg summer/41 winter
Engine swap build:http://geometroforum.com/topic/4850034/1/
Quote
Like
Share

1DCGUY
Elite Member
Joined: October 4th, 2008, 1:31 am

March 20th, 2011, 2:12 am #3

Very nice job!! :thumb

I just added this to my bookmarks. :rocker
Jeff Therring "METRO KING" AKA "Forum Soldier #1"

Home of the "Minnesota Metro Meet" August 11th 2018

Click > Here < for more Info
IC2


Quote
Like
Share

Wobblybob
Elite Member
Joined: January 29th, 2009, 12:21 am

March 20th, 2011, 3:55 am #4

Here is the same information in one marked up photo:



Last edited by Wobblybob on April 7th, 2011, 3:36 pm, edited 1 time in total.


"The Flying Egg" 3/5 and "Otto" 4/auto
Quote
Like
Share

work car
Elite Member
Joined: August 15th, 2010, 6:57 pm

March 20th, 2011, 4:19 am #5

man this is some good stuff! good job wobbly bob!
many metro project's never enough time!

all this metro madness leads to a drained wallet, unhappy wife, full driveway, confused unhappy neighbors, and more motors and transmissions to trip over, than you can shake a stick at!! but i got 50 mpg!!!
Quote
Like
Share

nerys
Elite Member
Joined: September 26th, 2009, 3:50 am

March 20th, 2011, 5:10 am #6

what if your LOOKING at a tranny already pulled from the car? I need to go get me a new tranny sadly he has two and I want to be SURE I am getting the 3.79 tranny and not the 4.10 tranny.
Quote
Like
Share

bogs
Elite Member
Joined: October 6th, 2009, 4:57 am

March 20th, 2011, 5:31 am #7

Very nice expansion to the method pacapo started some time back :thumb Also bookmarking this.

Red Green "Sound ingenious? Sound incredible?? Sound impossible??? Who cares!! I'm not listening :rasp "
. Mans Toolbox - You only need 2 tools, WD40 is for when it doesn't move but should, and Duct tape is for when it shouldn't move but does :rofl
When I'm not here, I'm taking up space on the net or planning to keep track of something :hmm
Quote
Like
Share

Tofuball
Elite Member
Joined: July 14th, 2010, 4:57 pm

March 20th, 2011, 5:56 am #8

This is GOLD.
Quote
Like
Share

Woodie
Elite Member
Joined: February 13th, 2009, 12:29 pm

March 20th, 2011, 9:05 am #9

Very nice. About time someone got around to doing this. Thanks from the entire community.
Alcohol, Tobacco, and Firearms
Should be the name of a convenience store, not a government agency.
Quote
Like
Share

Tofuball
Elite Member
Joined: July 14th, 2010, 4:57 pm

March 20th, 2011, 11:03 am #10

I ran those numbers through the engine RPM calculator http://www.csgnetwork.com/multirpmcalc.html
Assuming a 5th gear ratio of 0.757 and got:


Engine RPM with 12 inch tires (21.3 inch diameter):

@55MPH:
Swift 2312
XFi 2489
Metro 2693
Vert 2883

@70MPH
Swift 2942
XFi 3168
Metro 3427
Vert 3670

Engine RPM with 13 inch tires (22.8 inch diameter):

@55:
Swift 2160
XFi 2325
Metro 2516
Vert 2694

@70
Swift 2749
XFi 2960
Metro 3202
Vert 3428

EDIT: Remember factory tacho and speedo on most cars are not really very accurate. I don't think this would work to determine exactly what trans you have if you are only using factory gauges (it might?).
Last edited by Tofuball on March 20th, 2011, 11:10 am, edited 1 time in total.
Quote
Like
Share