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bandit66
New User
| Posts: 10
| Joined: 07/08
Posted: 07/17/08
01:32 PM
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My speedometer is not accurate. It works, it's just well below (around 10-15mph) the correct speed at all speeds. 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 (I6,Sprint 200,3.3L,120HP,Original owner) (C-4 Auto rebuilt,Autolite 1100 carb) (Power steering,manual drum brakes: F&R,9 x 2 1/4) (Factory single exhaust,7.25" rear,2.83 ratio,P205/75R14 tires) (White power top,Candy Apple Red exterior,Red interior).
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jlg2002
Enthusiast
| Posts: 449
| Joined: 03/08
Posted: 07/17/08
02:33 PM
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Yellow is supposed to be an 18T gear. They start at 16 and run thru 21 by color. see the NPD catalog pg 139 for more detail.
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bandit66
New User
| Posts: 10
| Joined: 07/08
Posted: 07/17/08
05:55 PM
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Ok. 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 (I6,Sprint 200,3.3L,120HP,Original owner) (C-4 Auto rebuilt,Autolite 1100 carb) (Power steering,manual drum brakes: F&R,9 x 2 1/4) (Factory single exhaust,7.25" rear,2.83 ratio,P205/75R14 tires) (White power top,Candy Apple Red exterior,Red interior).
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Posted: 07/21/08
07:26 AM
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More teeth = faster speedo. The big question may be how it became wrong first place. You might have a different gear set (ratio) in the diff than it had. Perhaps the whole rear axle might have been swapped. You might have a different tranny. My '65 had been converted to a C4 PRND21 from the Cruise o Matic (PRN2D1)it had. They didn't bother to check or change the speedo gear! After 42 years there could have been many repairs you don't know about.
As has already been mentioned, an 18 tooth sounds right. The rule of thumb is 1 tooth = 2 MPH. I had to try three gears before I got the right one.
NPD has 'em. Their catalog explains what you need to look for!
Harry
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