Speedo gear

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machattack

Well-known member
Joined
Apr 27, 2019
Messages
702
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Location
Idaho
My Car
1971 mustang mach 1 351c 4v auto fmx
1968 mustang gt 302 4v 4 speed
Hi,

I could use some help on this.  I changed the gearing in the axle to 3.50 gears. It came with 3.0 gears. Do I need to change the speedo cable gear now? 

Thanks for helping 

 
Yes most likely, here is formula that will get you close. Drive gear teeth X axle ratio X tire revolution per mile divide by 1001 = driven gear. An example most trans drive teeth are 6,7,8.

Here is my 73 

7x3.70= 25.9

25.9 x721.9=18697

18697 divided by 1001=18.69 round it up and your at 19 tooth driven gear.

Use this site to help determine revolution per mile depending on your tire size. https://tiresize.com/calculator/

Hopefully somebody has a shorter way of doing this and will share it here, its just what i use.

 
Simple method *if* your speedo was accurate prior to the swap. 

Divide 3.50 by 3.00 = 1.166

Multiply 1.166 times the number of teeth on your current driven gear - you need *more* teeth on the driven gear to slow the speedometer. 

 
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