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post #1 of 4 (permalink) Old 07-15-08, 04:12 PM Thread Starter
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Join Date: Jun 2001
Location: Virginia
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Speedometer not accurate...

1966 Mustang (Engine: 200 CID Straight-6, C-4 Automatic Transmission, Carburetor: Autolite 1100, Rear Axle: 2.83:1, Tires: P205/75R14, Style: Convertible, Color: Candyapple Red).
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Where do I locate and how do I remove the driven gear? I looked, but can't find it. I saw online that the 66 Mustang original tire size is 185/75-14. Is this correct? If so, I don't know if the speedometer was previously calibrated for that size or the current size (205/75/14). Thanks.

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post #2 of 4 (permalink) Old 07-15-08, 08:03 PM Thread Starter
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Re: Speedometer not accurate...

Speedometer is about 13-15 mph below what it should be.
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post #3 of 4 (permalink) Old 07-17-08, 04:03 PM Thread Starter
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Re: Speedometer not accurate...

I removed the driven gear. It's 16R and is light yellow. I believe 16 is the correct teeth based on the formula. I've heard that the drive gear is 7 and is specific to the C4 transmission. Is it possible that is not actually the one being used? If so, could that be the reason for the issue, can it be removed and where is it located? What are other possible sources of the speedometer problem?

Tires= P205/75R14
Differential Gear Ratio= 2.83
Rear Tire Diameter (Inches)= 26.11
Speedometer Revolutions/mile = 773
Drive Gear Teeth Number = 7

1966 MUSTANG Convertible (L6, Sprint 200, 3.3L, Original owner) (C-4 Auto rebuilt trans, power steering, manual drum brakes: F&R 9") (Single exhaust, 7.25" rear, 2.83 ratio, P205/75R14 tires) (Candyapple Red exterior, Red interior, White power top).
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post #4 of 4 (permalink) Old 07-17-08, 08:01 PM Thread Starter
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Re: Speedometer not accurate...

I drove the car and used a GPS device to show the actual speed I was driving. At 30 mph (GPS), the speedometer showed 20mph. At 40 mph> 30mph. At 60 mph> 45mph.

1966 MUSTANG Convertible (L6, Sprint 200, 3.3L, Original owner) (C-4 Auto rebuilt trans, power steering, manual drum brakes: F&R 9") (Single exhaust, 7.25" rear, 2.83 ratio, P205/75R14 tires) (Candyapple Red exterior, Red interior, White power top).
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