Jump to content

Figuring gear ratios in manual transmission


Recommended Posts

So I'm posting this in hopes datzenmike see it.  I have been looking at a post from years ago about changing ratios in the FS5W71 transmission.  I understand dividing the drive and driven gears to get the ratio in the middle.  What I don't get is how to come up with the final ratio I'm seeing on the right side.  Anyone able to help on this? Next question,  I have a 5 speed out of a 79 280zx.  Is it possible to change only the 5th gear ratio to something a little closer to the .745 5th in the later trans.  I know the synchronizer set up is different because mine has the odd ball 5th.  Anyway is it possible and what does it take if it is?

Screenshot_20220119-204044.png

Link to comment
  • Replies 11
  • Created
  • Last Reply

Top Posters In This Topic

Top Posters In This Topic

Posted Images

Box #1 is a mid ratio. Box #2 is a close ratio. As you can see the last gear sets (OUT) on the far right produce the OD (over drive) and they are the same 26/44 so swapping them does nothing. The left side (IN) will change every gear after it.

 

Box #1 IN is 31/22 or 1.409. Multiply 1.409 X any other gear to the right gives the output ratio. Example 1.409 X 0.591 = .8327 or rounded to 0.833 OD or 5th gear

 

Box #2 IN is 29/23 or 1.261 x the (OD 26/44) 0.591 = 0.745

 

There is no way to lift the 0.745 OD and place it into the mid ratio transmission. You could cut new gears with a 0.53 ratio something like 24/45 0.533 

 

 

BTW... I have used a '79 zx mid ratio for years in both my truck and my 710 and really liked the ratios. I have a wide ratio in my car now for a faster revving first gear.

  • Like 1
Link to comment

Thanks for getting back to me.  I'm just trying to make sure I know what I have.  Like I said it's supposed to be out of a 79 ZX.  The more I look I don't think I will be able to just change 5th since mine has the steel synchro 5th.  Correct me if I'm wrong on any of this.  My tooth count for 5th is 19 on the main and 31 on the counter shaft.  I havnt even seen this combo available when looking. Maybe it's because it is an odd ball and it's no longer available or wasn't commonly used so it's not available.  I'm building this trans to put in my MG just really wanted a little taller 5th to bring RPMs down but want to keep everything else the same because it's close to the original MG ratios.  Still would like to know how to correctly figure the final ratio for each gear if anyone can explain that.

Link to comment

Box #1

IN far left side, is 31/22 or 1.409. Multiply 1.409 X any other gear to the right gives the output ratio. Example 1.409 X 0.591 (OD 5th) = .8327 or rounded to .833 OD or 5th gear

 

Box #2

IN far left is 29/23 or 1.261 x 0.591 (OD 26/44 5th)= 0.745

 

 

Are you going to engine swap your MG to a L series engine?????

 

Link to comment

Thanks for the info on how to figure the ratios.  No keeping the MG engine, just wanted a 5 speed because it's geared to low to run interstate speeds.  There's a company that builds an engine plate that takes the Nissan trans.  No point in changing rear axle to a higher gear since I'm only working with 95hp so acceleration would suffer.

Link to comment

You may have to get a free picture hosting site account such as imgur. (there are others) You store your picture there till needed. Then copy and bring here and paste it in your post.

Link to comment
  • 2 months later...
On 1/19/2022 at 7:22 PM, Mough said:

Is it possible to change only the 5th gear ratio to something a little closer to the .745 5th in the later trans.  I know the synchronizer set up is different because mine has the odd ball 5th.  Anyway is it possible and what does it take if it is?

Yes, it is possible to change only the fifth gear ratio. Unfortunately, since you have the crush type syncros, there are only two ratios available, the common one (31x19 teeth, 0.613 ratio) and the rare one (38x22 teeth, 0.579 ratio). I sold both of my rare sets for $285 each in 2013 & 2014, so I cannot help you there.

Link to comment
On 1/20/2022 at 1:49 PM, datzenmike said:

19/31 is strange for the OD gears. This is a 0.6129 X the input of 1.409 = 0.863 OD

 

Can you post a picture of your transmission???

Not strange for transmissions with crush type synchros. I have sold dozens of sets in the last 40 years and still have 6 sets in stock.

Link to comment

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...
×
×
  • Create New...

Important Information

By using this site, you agree to our Terms of Use.