tire size problem? speedo problem?
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Elder User
Joined: Nov 2010
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From: lincoln, NE
tire size problem? speedo problem?
ok guys my speedo doesn't read right, when it reads 37 im doing 45 when it reads 40 i'm doing 52 and when it reads 45 i'm doing 60 it didn't read at all when i got the truck and i replaced the gear in the transfercase (there was one left from my dads old bronco) so do u guys have any ideas as to what could be causing this problem? just fyi i've got 28" tires on it, and it's a 91 f150, 300 I6 4 speed 4x4
FTE Legend
Joined: Jun 2006
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From: Ottawa, Ontario
Sounds like you put the wrong gear on the cable, there is a selection of driven gears available with different colors for a different number of teeth, and the one you need will depend upon the axle gearing and tire size.
Ford Speedometer Gears for Automatic and Standard Transmission
To figure out what gear you need use this formula..
Speedometer Gear Teeth = Drive Teeth * Rear End Ratio * 20.2 / Tire Height
Ford Speedometer Gears for Automatic and Standard Transmission
To figure out what gear you need use this formula..
Speedometer Gear Teeth = Drive Teeth * Rear End Ratio * 20.2 / Tire Height
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Elder User
Joined: Nov 2010
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From: lincoln, NE
see i dont think it's a gear problem, because if the gear was wrong i would be off by the same amount at every speedo reading but it's not it goes everywhere from being 7 mph off to being 15 mph off
Cargo Master
Joined: Jan 2004
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From: South Dakota
If you have a gear with the wrong number of teeth, which I am sure you do, your speedometer will be off by a relative amount or a certain percentage depending on speed.
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Elder User
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From: lincoln, NE
but it wasnt the same as the ones you posted in this picture it was the big one that went around the rod
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Cargo Master
Joined: Jan 2004
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From: South Dakota
OKAY, that is NOW understood. You will still need to do some swapping of the gears that conanski posted a picture of to correct your speedometer reading. I just hope they make a big enough variation of the number of teeth to make your proper correction. For instance if you have 20 teeth on your current gear, you would need, I would guess, a 15 or 16 tooth gear to correct it.
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Elder User
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From: lincoln, NE
but the thing is i cant possibly see that gear being the wrong number of teeth because i know from the guy who had it before me that he never changed that gear nor rear gear ratios nor tire size so it cant be a problem with number of teeth on the gear can it?
Cargo Master
Joined: Jan 2004
Posts: 3,118
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From: South Dakota
Well, did you check that the gear you replaced on the transfer case shaft had the same number of teeth as your old worn out gear? Either way the easiest way to correct your speedometer is switch the gears like conanski posted. Again this is if the available gears will allow you to do enough correction.
Cargo Master
Joined: Jan 2004
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From: South Dakota
Then it would seem that the speedometer was always off. Regardless of whether the speedometer was always off, the answer remains the same"swap out the plastic gears like those conanski posted".
Cargo Master
Joined: Jan 2004
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From: South Dakota
You don't need to worry about what your gear ratio is. You already know how fast the truck is going compared to your speedometer reading. Pull your speedometer cable out of your transfer case and count the number of teeth on that plastic gear. Now use the math your teacher taught you to figure out how many teeth you will need on your replacement gear. Just so you won't be confused, it will need to be LESS teeth than you have now(see #9 in this thread). This should get you pretty close, if you want it perfect you may have to buy a second gear with one tooth difference more or less.
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