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Old 04-01-2002, 02:42 PM   #1
Kamaro Killer
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Default how can you tell what size gears you have?

I have the problem with my speedo as most people here, I have bigger gears in the diff, but dont have the speedo gear to let my speedo know that I have bigger gears. I want to buy the right gear for my speedo, but since I bought my car after the gears were installed, I don't know what size the gears are. I'm pretty sure they are 3.73, but I dont want to buy the wrong gear. Is there any way of being able to tell, and if so how?

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Old 04-01-2002, 03:30 PM   #2
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well there are a couple of ways.

1. take your diff cover off and count the teeth on the ring and pinion, and then divide them to come up with the gear ratio.

2. put your car on jack stands with it in gear and make a mark on your driveshaft. Spin one of the rear tires one entire revolution and watch the drive shaft to tell how many revolutions it makes during the one tire revolution. If the driveshaft spins almost four complete times during the tire rotation, then you have 3.73's.
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Old 04-01-2002, 07:03 PM   #3
GodStang
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It should say on a tag on your axle.
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Old 04-02-2002, 12:34 AM   #4
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Quote:
Originally posted by DRASTiK
2. put your car on jack stands with it in gear and make a mark on your driveshaft. Spin one of the rear tires one entire revolution and watch the drive shaft to tell how many revolutions it makes during the one tire revolution. If the driveshaft spins almost four complete times during the tire rotation, then you have 3.73's.
This works well, but put the tranny in neutral, rather than in gear.

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Old 04-02-2002, 11:40 AM   #5
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Quote:
put the tranny in neutral, rather than in gear

oops. hehe it's been a while since I did this. Didn't mean to mislead you.
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Old 04-02-2002, 02:12 PM   #6
Mr 5 0
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Lightbulb Name that gear

Quote:
Originally posted by GodStang

It should say on a tag on your axle.
There's also a door tag with a code but if you have aftermarket (non-stock) rear gears, all that information is useless.

Use the 'marked driveshaft/one tire rotation' method to determine rear diff ratios, especially on older Mustangs that very often have aftermarket gears installed as this is a basic Mustang mod.
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Old 04-03-2002, 02:31 PM   #7
james87gt
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i just got done and the driveshaft passed 3 1/2 times what size grears do i have i would think 355 if four full truns is 373
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Old 04-03-2002, 02:55 PM   #8
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3.5 rotations = 3.55
3.75 rotations = 3.73
4 rotations = 4.10 or possibly a 3.90, but I'm not aware of a 3.90 gear available for the 8.8 rearend.
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