Speedo gear suggestions
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Lead Driver
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From: Greater Austin, Texas
Speedo gear suggestions
I converted my F150 to a ZF trans and put in a 3.08 rear-end. I need a speedo gear with 15 teeth to make my speedo correct. Ford says they only go down to 16 teeth (that's the one I have now). The Ford guy said maybe some other manufacturer uses the same gear and they might have one? Longshot. Has anybody else run into this and what did you do? I am thinking I may have to put in a reducer and then gear to that or perhaps replace the worm gear on the output shaft but wanted to see if anyone here had a better idea. Thanks for any suggestions.
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Joined: Aug 2005
Posts: 2,760
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From: Gadsden Purchase
BTDT with my '75 with 3.25 and T18 transmission and 235/85-16 tires.
Ford etc doesn't make a 15, smallest is a 16 and i had a 17.
I decided not to go with a 16... just test and calculate the speed and odometer.
The odometer was off 12.9% and had to be factored-in to calculate the MPG.
I adjusted the speedometer (yeah they are sorta-adjustable) to read as fast as i could get it to read and was shy about 5mph at about 65mph.
Lately i switched to 3.50 R&P and that fixed my speedometer, so it's now right on the money. :)
First time since '77 BTW. :)
Haven't messed with testing my odometer again yet, just got it calculated-out so far.
Anyway, I've decided to keep my 17 tooth when a 16 tooth would fix the odometer just about perfect.
Seems to me at this point consistency is more important than accuracy, anyway.
I've been living with the 17 for 32+ years might as well keep it. :)
A guy could prob'ly adjust the speedometer to read right if the odometer was reading right.
Otherwise they may not be the same anyway.
What do you think? :)
What size did you have to start with? :)
http://www.panix.com/~alvinj/file12/SpeedTable.jpg
(from the back of an SPRR "time table")
Alvin in AZ
Ford etc doesn't make a 15, smallest is a 16 and i had a 17.
I decided not to go with a 16... just test and calculate the speed and odometer.
The odometer was off 12.9% and had to be factored-in to calculate the MPG.
I adjusted the speedometer (yeah they are sorta-adjustable) to read as fast as i could get it to read and was shy about 5mph at about 65mph.
Lately i switched to 3.50 R&P and that fixed my speedometer, so it's now right on the money. :)
First time since '77 BTW. :)
Haven't messed with testing my odometer again yet, just got it calculated-out so far.
Anyway, I've decided to keep my 17 tooth when a 16 tooth would fix the odometer just about perfect.
Seems to me at this point consistency is more important than accuracy, anyway.
I've been living with the 17 for 32+ years might as well keep it. :)
A guy could prob'ly adjust the speedometer to read right if the odometer was reading right.
Otherwise they may not be the same anyway.
What do you think? :)
What size did you have to start with? :)
http://www.panix.com/~alvinj/file12/SpeedTable.jpg
(from the back of an SPRR "time table")
Alvin in AZ
Last edited by Alvin in AZ; Jun 22, 2007 at 12:35 AM.
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Lead Driver
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From: Greater Austin, Texas
After several unsuccessful attempts to find a 15 tooth driven gear, I have ordered an 8-tooth drive gear to replace my 6-tooth. The drive gear on the ZF just slides on the output shaft and is held in place by the yoke, so I don't have to remove the rear extension housing to replace it. This combined with a 20 tooth driven gear should give me the right setup.
I have been recalculating the odo ever since I put in the 3.08 gear because I knew the 16 tooth gear I put in wasn't enough, but it was close. Only 3.3% off on the speedo but recently found out my odo was 6.6% off so I am calibrating to that. That will make the speedo be 3.3% fast but that is OK, better than being slow.
I have been recalculating the odo ever since I put in the 3.08 gear because I knew the 16 tooth gear I put in wasn't enough, but it was close. Only 3.3% off on the speedo but recently found out my odo was 6.6% off so I am calibrating to that. That will make the speedo be 3.3% fast but that is OK, better than being slow.
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Postmaster
Joined: Aug 2005
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Likes: 8
From: Gadsden Purchase
Originally Posted by jas88
...ordered an 8-tooth drive gear to replace my 6-tooth.
That was never mentioned as a fix for my T18 (back when). :/
...will make the speedo be 3.3% fast but that is OK, better than being slow.
It's simply seen as "how much spring pressure" the needle has against its stop.
But 3.3% is what? ...two mph high at 65? :)
Mine was easy to adjust since i was going the other direction... i adjusted it for the least amount of pressure-that-would still zero the needle and called it good. :)
It was like 8% off after that ...rather than the 10 to 12% it was off before. :)
Alvin tinkering in AZ
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