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Discussion Starter #1
One thing I didn't consider when I did the C4 upgrade was the speedometer. Its now way off. (my fault) The shop that rebuild my C4 is helping me fix this. They asked for the rear end ratio and tire height.
I know its an open diff, (tiny little thing for sure) I supported the the rear of the car on jacks, marked the drive shaft to count rotations and did the same for the driver side rear wheel. I blocked the pass rear so it would not rotate.
one tire rotation gave me about 1.75 drive shaft rotations, If I take 1.75 x 2 = 3.5. So my rear gearing is 3.5? Does this sound about right for a 1960 Comet?

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Is the axle stock?
If so there are 4 choices.
3.10
3.50*
3.56
3.89
The axle code on the warranty plate on the door or door jamb will have the data.
If the axle code is a number it is a standard axle. If a letter* it is a limited slip

3.50 is a choice and that should be limited slip

1960 Mercury Comet VIN Identification (mercurystuff.com)

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Discussion Starter #3
Is the axle stock?
If so there are 4 choices.
3.10
3.50*
3.56
3.89
The axle code on the warranty plate on the door or door jamb will have the data.
If the axle code is a number it is a standard axle. If a letter* it is a limited slip

3.50 is a choice and that should be limited slip

1960 Mercury Comet VIN Identification (mercurystuff.com)

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Thank you Action!! That's super cool info.. I learned a lot about my car with this decoder. My tag says Axle 1 which is a 3.56. Is this a stock unit? I don't know, The person i bought it from said she had it rebuilt so I don't know if its the original or not.. however, my math came up with a 3.5, so I'm very close. The code also said my car was built in Lorain, OH, its (was) Inlet Blue, Build July 1st, 1960, a Friday built car. I can imagine on the assembly line, all the workers were thinking about the weekend and the upcoming 4th of July!

Thanks again,

Cheers
 

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One thing I didn't consider when I did the C4 upgrade was the speedometer. Its now way off. (my fault) The shop that rebuild my C4 is helping me fix this. They asked for the rear end ratio and tire height.
I know its an open diff, (tiny little thing for sure) I supported the the rear of the car on jacks, marked the drive shaft to count rotations and did the same for the driver side rear wheel. I blocked the pass rear so it would not rotate.
one tire rotation gave me about 1.75 drive shaft rotations, If I take 1.75 x 2 = 3.5. So my rear gearing is 3.5? Does this sound about right for a 1960 Comet?

Thanks
When you say "way off", how far off is that?
 

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currently run a stock 3.50 rear in a '63 with a T5 Overdrive tranny and a Maverick/T5 with a 2.89 rear.

You can swap out the driven speedo' gears. A higher numerical - lower gear ratio driven speedo gear for a closer match to your actual speed for higher numerical-lower ratio rears. Tire size also affects finla ratio.


Simple 'archaic' GPS display or a discrete cheapo' GPS speedo will provide fairly accurate speed for comparison or confidence of speed.

have fun

.
 

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Rule of thumb
to add or subtract a driven gear is 7MPH per gear fewer teeth will speed up the speedometer

more teeth will slow down
 

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Discussion Starter #7
When you say "way off", how far off is that?
Hi! Sorry it took so long to reply.. Long weekend, was off the computer.
Its off by quite a bit.. guessing 60mph and the speedo reads 90-95mph (LOL, that car has never seen 90) But in thinking about the used transmission i bought, the seller did tell me it came out of a 6cyl mustang.. So Didn't Mustangs of that era come with 14" tires? my Comet has tiny 13" tires so maybe that has something to do with it? Not sure. The shop who rebuilt that C4 are helping me fix this. I was told to measure the tire height and crawl underneath the car and locate the speedo cable hole, mark the gear and count.
 
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