new gears

SPAIN72MUSTANG

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hi i just replaced the gears with 3.80 gears and i need the speed gears or the speedometers thats attaches to the tranmission,does any one no what size tooth i need for the mustang is 8 inch rear end with 28 spine and 15x7 tires i need help ordering the right one and i dont know what size i need;thanks ron

 
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It requires some additional information. I need actual tire size, not just wheel size and the type transmission in your vehicle. It also helps to know what gear ratio you had before, if the speedometer was right and what color the current gear on the cable is.

The output shaft has the driven gear which could be 6,7 or 8 teeth depending on the transmission and automatics and manuals use a different gear.

I can steer you to a calculator too

http://www.sccoa.com/faq/speedgr.html

 

SPAIN72MUSTANG

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jeff73mach1 tire size 235/65/15 c4 transmission and 2.83 i think its 7 teeth from what i read hope you can help thanks ron

 
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Here's your gear if your info is correct it should put you within 1% of accurate. Stated Tire size and actual tire size vary between brands, runs etc. That is as close as you are likely to get.

http://www.summitracing.com/parts/tci-881002/applications

If your tire size is wrong, for the sake of argument and is actually 235-60-15 that little bit of error adds an inch to the tire height and introduces about a 3.7% error In that case a 21 tooth gear would be closer to perfect.

All that is nice and warm and fuzzy-here is the not so nice news

C-4's drive gear is cut into the output shaft of the transmission and it could be 6, 7, 8, or 9 teeth

If your car came with a 2.83 gear originally and the speedometer was correct then it should have had a 14 tooth gear-but ford starts with 16 teeth meaning they would use 8 teeth cut into the shaft to get a 16 tooth gear to work with high (numerically low) ratios

If you have a 8 tooth shaft you will need a 23 tooth gear. This is not a factory original gear. The tooth pitch is a bit off so they wear out ever 15-25K miles.

here is that part http://www.summitracing.com/parts/tci-881004/overview/

if you would count the teeth of the current gear I could guide you as to which one would be most likely to meet your needs

 
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Don C

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the problem with those is they don't account for tire size. If you run a stock diameter tire, then they work okay, but he has an in between gear and a tire size that is not original (It's a little taller) so it requires doing the math to get it right.

 

Don C

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the problem with those is they don't account for tire size. If you run a stock diameter tire, then they work okay, but he has an in between gear and a tire size that is not original (It's a little taller) so it requires doing the math to get it right.
The calculator on the left side of the charts takes into account the drive gear teeth, the tire diameter, and the axle ratio to calculate the number of teeth needed for the speedometer gear.

 
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