Speedo Gear
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Fleet Mechanic
Joined: Oct 2007
Posts: 1,972
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Speedo Gear
Based on the information I have and don't have, can I determine which speedo gear I need?
I have edited this to indicate a 3.10 rear gear. Whether this is true or not, I can not say. The tag on the rear end definitely has "3.10" stamped on it, and nothing else.
Tire diameter: 29.3 in
indicated speed: 50 mph
actual speed: 67 mph
292/COM 1:1
ring and pinion: 3.10
rpm: unknown
I have edited this to indicate a 3.10 rear gear. Whether this is true or not, I can not say. The tag on the rear end definitely has "3.10" stamped on it, and nothing else.
Tire diameter: 29.3 in
indicated speed: 50 mph
actual speed: 67 mph
292/COM 1:1
ring and pinion: 3.10
rpm: unknown
Tuned
Joined: Feb 2009
Posts: 370
Likes: 0
From: Near Nashille, TN
It's about percentages.. The driven gear needs to be smaller to spin faster by the same percentage that the speedo is off.
If you happen to have a 22 tooth gear: 50/67 = .74
You're indicating .74 of actual speed, so you need a driven gear that is .74 of your current gear.
.74 * 22 = 16.28 tooth needed. So 16 or 17 tooth is what you'd need in this case.
hiball is right.. that is a big difference. Do you happen to know the tooth count of the speedo gear?
If you happen to have a 22 tooth gear: 50/67 = .74
You're indicating .74 of actual speed, so you need a driven gear that is .74 of your current gear.
.74 * 22 = 16.28 tooth needed. So 16 or 17 tooth is what you'd need in this case.
hiball is right.. that is a big difference. Do you happen to know the tooth count of the speedo gear?
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Fleet Mechanic
Joined: Oct 2007
Posts: 1,972
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I do not know how many teeth the gear has. My warranty plate was missing so I do not know what gear my truck had when it left the factory.
I searched around the web and found <ahref="http: ninecase.htm"="" www.kevinstang.com="">this link. The first picture of a gear tag looks like mine.</ahref="http:>
http://www.kevinstang.com/Ninecase.htm
<ahref="http: ninecase.htm"="" www.kevinstang.com=""><ahref"<a<a target="_blank" href="http://www.kevinstang.com/Ninecase.htm>this">
Based on that info, I have a 3.10 rear gear out of a 57 or 58 Ford.</ahref"<a</ahref="http:>
I searched around the web and found <ahref="http: ninecase.htm"="" www.kevinstang.com="">this link. The first picture of a gear tag looks like mine.</ahref="http:>
http://www.kevinstang.com/Ninecase.htm
<ahref="http: ninecase.htm"="" www.kevinstang.com=""><ahref"<a<a target="_blank" href="http://www.kevinstang.com/Ninecase.htm>this">
Based on that info, I have a 3.10 rear gear out of a 57 or 58 Ford.</ahref"<a</ahref="http:>
Ford Parts Specialist
Joined: Nov 2006
Posts: 88,826
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From: Simi Valley, CA
The Warranty Plate does not have speedo gear info. What is has...that pertains to speedo gears...is the TRANS & AXLE codes.
In order to figure out which DRIVE and DRIVEN speedo gears are correct, I need the year of the truck, trans type, rear axle ratio and the tire size.
The DRIVEN gear is located at the end of the cable where it fits into the trans. Could be nylon, cast iron or steel w/a fiber gear riveted to the end of it.
The DRIVE gear is located on the output shaft, could be nylon or steel.
No 1957/83 F100 w/a 9" came w/a 3.10 ratio. No 1957/59 Ford Passenger Car & Thunderbird (9" only rear axle available) did either.
But if your truck is an F100, it may not have a 9. Look an the back of the housing to see if it has an inspection cover that was only used with Spicer/Dana rear axles with Limited Slip.
In order to figure out which DRIVE and DRIVEN speedo gears are correct, I need the year of the truck, trans type, rear axle ratio and the tire size.
The DRIVEN gear is located at the end of the cable where it fits into the trans. Could be nylon, cast iron or steel w/a fiber gear riveted to the end of it.
The DRIVE gear is located on the output shaft, could be nylon or steel.
No 1957/83 F100 w/a 9" came w/a 3.10 ratio. No 1957/59 Ford Passenger Car & Thunderbird (9" only rear axle available) did either.
But if your truck is an F100, it may not have a 9. Look an the back of the housing to see if it has an inspection cover that was only used with Spicer/Dana rear axles with Limited Slip.
Postmaster
Joined: Apr 2002
Posts: 3,613
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From: Coal country
It is pretty easy to count the teeth on the driven gear, which should be plastic.
It also is often imprinted on the gear itself, if you look at the face of the gear.
Find the existing tooth count, multiply it by 0.74 as mentioned, and that will tell you the tooth count you need based on your existing tire size and rear end ratio.
Finally, they are cheap enough - if you go to the local dealer, anyway. The same gear was used in multiple Fords up to about 1996, if I remember correctly, so they are still a dealer item. Recently, I got a package of 2 for just about $5. They had the size I wanted in stock, which was 17. Cheap enough to experiment with anyway.
It also is often imprinted on the gear itself, if you look at the face of the gear.
Find the existing tooth count, multiply it by 0.74 as mentioned, and that will tell you the tooth count you need based on your existing tire size and rear end ratio.
Finally, they are cheap enough - if you go to the local dealer, anyway. The same gear was used in multiple Fords up to about 1996, if I remember correctly, so they are still a dealer item. Recently, I got a package of 2 for just about $5. They had the size I wanted in stock, which was 17. Cheap enough to experiment with anyway.
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Fleet Mechanic
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Postmaster
Joined: Apr 2002
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From: Coal country
You have a Ford 9" rear axle.
You might drip a bit, but no gush - unless the transmission is way overfull.
Obtain your rear axle ratio this way:
Jack up one rear wheel, ensure the other side will not rotate.
Put the truck in neutral.
Rotate the wheel one revolution while counting the number of revolutions the driveshaft makes.
If the driveshaft turns about 3 and 1/4 rotations while the wheel goes around only once, then your ratio is 3.25 to 1.
Two turns plus an almost complete third turn would make it a 2.75 to 1.
(I have not included the obvious safety steps, to avoid wordy clutter.)
You might drip a bit, but no gush - unless the transmission is way overfull.
Obtain your rear axle ratio this way:
Jack up one rear wheel, ensure the other side will not rotate.
Put the truck in neutral.
Rotate the wheel one revolution while counting the number of revolutions the driveshaft makes.
If the driveshaft turns about 3 and 1/4 rotations while the wheel goes around only once, then your ratio is 3.25 to 1.
Two turns plus an almost complete third turn would make it a 2.75 to 1.
(I have not included the obvious safety steps, to avoid wordy clutter.)
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Fleet Mechanic
Joined: Oct 2007
Posts: 1,972
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Is it possible to remove the speedo needle and put it back on clocked wrong?
Is this just crazy talk, or could the PO have done that?
If so, I could remove the glass, pull the needle and use my gps to reset the needle.
Is this just crazy talk, or could the PO have done that?
If so, I could remove the glass, pull the needle and use my gps to reset the needle.
Logistics Pro
Joined: Mar 2005
Posts: 3,771
Likes: 80
From: tujunga, calif
I'll look for Number Dummy to address this but I seem to recall many moons ago changing the drive gear on one car trans before I could get the speedo close. Don't know if there is another that would fit your trans.
Tuned
Joined: Feb 2009
Posts: 370
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From: Near Nashille, TN
Yes, it's possible the needle came off at some point... I actually had that happen to me, and I was just lucky that I got it back on in the right spot.
It would be interesting to see how the mph compares to the odometer miles.
You can check the odometer by running down the interstate a few miles (using mile markers) and compare that to what your odometer rolls. Going 10 miles makes the math easy, but you can do it in less.
It would be interesting to see how the mph compares to the odometer miles.
You can check the odometer by running down the interstate a few miles (using mile markers) and compare that to what your odometer rolls. Going 10 miles makes the math easy, but you can do it in less.