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Discussion Starter · #1 ·
car in question is a 68 Mustang....

it has a C4 transmission, 9 inch rear end , and will be getting 3.50 gears....

what speedo gear to get? (part number would be very helpful too)

I dont know what size tires that are going to be used if it makes a big difference?

thanks in advance....
 

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What is/was the speedo and rear end gearing before you made/make the change?
Tire size does play a role in determining final gearing for the speedo, so I'd wait until I had that before doing anything.
You can always start out with what you had before and go from there.
The way I checked my speedometer was I found a five mile stretch of freeway with the speedometer check mileage posts. Of course, you're in Canada, so would it be kph?
I started out with my front end right even with the 0 post and noted where my odometer was, started out and went five miles, then ended with my front end as close to the same position at the five mile post as when I started out, checked my odometer and noted where it was.
Did the math and took that to my local speedo/instrument repair shop and they took my cable and added a reduction gear in the line to slow my speedo down.
 

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Discussion Starter · #3 ·
the car is not mine, belongs to someone I know....it currently has 4.11 gears in it now...what it has for a speedo gear I really dont know
 

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If you're going to go with tires that are drastically shorter or taller than the original size, which would be around 25.5 - 26" tall, then yes, it will make a difference for the speedo gear you need.

Is the C4 the original unit for the 68? I ask because some years of Ford trannys have a different tooth count on the drive gear on the output shaft.


cheers
Ed N.
 

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If the car has a tach you can use an online calculator and figure out what mph the car will be running at a certain rpm.From there you will be able to figure out how far out the speedo is and make the necessary changes.The size of the tires is very important to the final calculations though.
 
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