Vintage Mustang Forums banner

1 - 6 of 6 Posts

·
Premium Member
Joined
·
2,294 Posts
Discussion Starter #1
Last year, when I switched to the 9 inch rear, I installed 4.10 or 4.11 gears in the rear. Well, my old 8 inch had 2.79 gears in it. I know when I hook my speedo back up... (installing autometer 160MPH) its not going to read right. So what do I need to do to fix this? I have a C4, and from what Im told, they don't make new speedo gears for different ratios... they were all the same. So what option does this leave me?
 

·
Registered
Joined
·
2,204 Posts
You can get the correct gear from your favorite vintage mustang parts warehouse!

Go here: CJ Pony Parts Catalog And search for "speedometer gear"
 

·
Registered
Joined
·
6,008 Posts
The formula to use is:

Driven Gear teeth = (# of drive gear teeth) x (axle ratio) x Tire Revolutions per minute)/1000

# of drive gear teeth should be 7 for original 2.79

Tire revs/min = 63360/(Tire OD in inches) x 3.14159

This info is in the NPD catalog and they also have a variety of gears. For C4 you need a RH rotation gear,

I did this with mine and am within 3-4 mph,

Frank
 

·
Premium Member
Joined
·
11,700 Posts
I took my ride to the local speedo shop and they did it for me. My reasoning was that they could put it on a dyno and provide me with a card with speed numbers for each 10 MPH up to 80, which shows dead-on-accurate. That gave me a huge sense of security on the road and just might save my fanny a ticket someday.

Yes, I could have changed gears until I got it close, but that is all I'd ever get; close.
 

·
Registered
Joined
·
4,885 Posts
If you just need to correct for the gear swap then the easy formula is new ratio over old ratio times the number of teeth on your original speedo gear. My 2.79/C4 had an 18 tooth gear (they all don't have 7 tooth gears as stated above). My swap was from a 2.79 to a 3.55. 3.55/2.79 = 1.2724 x 18 teeth = 22.9032. I replaced the 18 tooth with a 23 tooth and the speedo is now roughly correct.
 

·
Registered
Joined
·
412 Posts
I sent a speedo gear calulator to your email address. This will help you determine the gear you need in the trans which you can get from NPD or others.
 
1 - 6 of 6 Posts
Top