slow speedo
Ford Parts Specialist
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From: Simi Valley, CA
16T DRIVEN gear is it, but the DRIVE gear on the output shaft should also be taken into consideration. What is the rear axle ratio?
Ford Parts Specialist
Joined: Nov 2006
Posts: 88,826
Likes: 787
From: Simi Valley, CA
Originally Posted by reddnekk
My 1966 F100 has a 68 390, 1967 Borg Warner 
4 speed.
4 speed."Tail shaft" marked: C7OR-7A040-A = Ford part number: C6OZ-7A039-C .. Extension Housing ~ Ford 4 Speed Manual "top loader" / Applications: Misc 1966/69 Ford/Merc Passenger Cars w/289/302/351W/390.
Sorry, there's nothing I can do. Ford 4 speeds were not installed in trucks, so there's no application charts for speedo gears listed in Ford truck parts catalogs.
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slow speedo
Thank You for trying! I know you will be of great help in the future. Thanks again. 
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Laughing Gas
Joined: Dec 2010
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From: Tucson, Arizona
Take your truck to a speedometer repair shop who has knowledge of classic vehicles. You may be able to come up with a speedo DRIVE gear, the one on the trans output shaft, that will make up the difference. Failing that, there are gearing correction boxes that can get you closer. These do not come cheap, however. If all else fails, buy a portable GPS nav system which will not only tell you your speed, more accurately than your speedometer, but other useful info as well. Good luck!
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Brett Foote
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From: Montpelier, VA
Assuming that your speedometer is good condition, there are 4 variables that come into play.
1) Drive gear on output shaft
2) driven gear on speedometer cable
3) rear end ratio
4) tire diameter
You didn't mention what size tires you have. Oversize tires will make the speedometer read lower than actual speed. Verify the rear end ratio with a helper by rolling the truck slowly on level ground and count the number of revolutions of the drive shaft for one revolution of the wheel. The tag may say 3.25:1 but it could have been replaced with something else in the past 46 years. The drive gear on a toploader is usually 6 or 7 teeth (dependent on original rear end ratio of the vehicle it came out of). The tailshaft needs to come off to change the drive gear. Here is a reference formula that may help you:
Speedometer Calibration Program - Method 1
1) Drive gear on output shaft
2) driven gear on speedometer cable
3) rear end ratio
4) tire diameter
You didn't mention what size tires you have. Oversize tires will make the speedometer read lower than actual speed. Verify the rear end ratio with a helper by rolling the truck slowly on level ground and count the number of revolutions of the drive shaft for one revolution of the wheel. The tag may say 3.25:1 but it could have been replaced with something else in the past 46 years. The drive gear on a toploader is usually 6 or 7 teeth (dependent on original rear end ratio of the vehicle it came out of). The tailshaft needs to come off to change the drive gear. Here is a reference formula that may help you:
Speedometer Calibration Program - Method 1
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