Speedometer gears????
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Cross-Country
Joined: Sep 2007
Posts: 92
Likes: 4
Speedometer gears????
Ok, so A few weeks ago I was asking questions about changing the gear ratio in my trucks rear end. Well I ended up doing it and I went from a 3.08 to a 3.73 ratio in my F150. One thing I neglected to consider was the speedometer being thrown off. it is currently reading about 10 mph faster than actual speed. (65mph on speedometer is actually 55mph). SO I am wondering now how do I figure out what speedometer cable gear I need??? MY current gear is a Pink 19 tooth right hand rotation. Also can I change just the speedometer cable gear and get a close reading or do I have to change the gear in the transmission as well?? Learning as I go here so any help is greatly appreciated.
Laughing Gas
Joined: Feb 2010
Posts: 781
Likes: 8
From: Sanford, NC
I changed the rear gear from a 3.25 to a 3.70 in my 1981 F100 with a nine inch rear and
a AOD trans.
I had to use the 23 tooth driven gear to get the speedometer to read right. Not sure
what other transmissions this gear will fit. You might check with Baumann Engineering
in South Carolina who sells this gear.
a AOD trans.
I had to use the 23 tooth driven gear to get the speedometer to read right. Not sure
what other transmissions this gear will fit. You might check with Baumann Engineering
in South Carolina who sells this gear.
Thread Starter
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Cross-Country
Joined: Sep 2007
Posts: 92
Likes: 4
Ok, Should have mentioned this earlier. MY transmission is a three speed with overdrive manual (4th gear being overdrive). This may be a really dumb question but from what I have read its not quite clear. Will I need to change the gear on the end of speedometer cable as well as the gear it meshes with in the transmission?? Some places have mentioned only having to change the gear on the speedometer cable? Still trying to figure out what set up I might need as well? Currently have Pink 19 tooth. thanks
Posting Guru
Joined: Jun 2004
Posts: 1,092
Likes: 2
From: Casa Grande
Changing it to a 21 tooth driven gear on the speedometer cable should put you about 3-4 mph off. You would actually need a 22 tooth gear but they don't make one. To get any closer you will have to go up, or down can't remember for sure, 1 tooth on the drive gear inside the trans.
FTE Legend
Joined: Jul 2010
Posts: 32,875
Likes: 48
From: Northeast, OK
Try searching in the forum for "speedo calibration" I did and turned up a thread that had this:
Do not change the gear in the transmission if and only if the Odometer is accurate.
Changing the Speedometer cable gear will affect both.
The speedometer head itself will need to be calibrated. There is a spring on the inside that needs to be adjusted to bring the needle speed back down to where it needs to be. But do this only if the Odometer is Accurate. The spring is on the needle shaft, and removing the needle and faceplate from the speedometer head may be needed to gain access to the spring. Turning the spring one way speeds the needle up, and turning it the other way slows it down. It's going to be hard to adjust the speedo yourself, as it's going to be trial and error.
There are speedometer shops that will calibrate and repair speedometer heads, for a fee out there too.
Of course if both the speedo, and odometer is off, then changing the speedometer cable gear is the correct thing to do.
Changing the Speedometer cable gear will affect both.
The speedometer head itself will need to be calibrated. There is a spring on the inside that needs to be adjusted to bring the needle speed back down to where it needs to be. But do this only if the Odometer is Accurate. The spring is on the needle shaft, and removing the needle and faceplate from the speedometer head may be needed to gain access to the spring. Turning the spring one way speeds the needle up, and turning it the other way slows it down. It's going to be hard to adjust the speedo yourself, as it's going to be trial and error.
There are speedometer shops that will calibrate and repair speedometer heads, for a fee out there too.
Of course if both the speedo, and odometer is off, then changing the speedometer cable gear is the correct thing to do.
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