Speedometer drive gear

6

62pan

Senior Member
I purchased a 39 1/2ton pickup last year. I just got it where I can start doing test drives and I noticed the speedometer was off and the odometer did not work. The speedometer is now reading half of whatever speed I am doing. The rear is a 4.11 with the correct drive in the turtle. Speedometer shop says I have incorrect speedometer for the drive on the drive shaft. Did they make two different speedometer drives that go on the driveshaft or were there different ratio speedometer, 2-1 or 1-1. From photos I found my speedometer does look like 39. Speedometer shop is well known so I'm at a loss.
 
Have you checked the diff. ratio? Being a PU you could have 4:44 ratio. Yes there are a few different speedo turtles for the torque tube speedo drive based upon rear gear ratio and tire size. 2-1 NO. The original speedometers are very much standard. The trip odometer will not work if the reset knob is in the reset position. If your regular odometer does not work there may be stripped gears.
 
Speedometer has been repaired and odometer works. Rear is definitely 4.11 Speedometer and odometer only read half of what I going using GPS as a guide. Shop swears its calibrated correctly. So where's the problem?
 
You need to verify that the turtle on the drive shaft ( speedo drive gear) is correct for your tire size and 4.11 ratio. There's a number on the turtle, and a chart to tell what number to use. I have never heard of one reading half speed though
 
Don’t assume the turtle gear is correctly identified by the number on the shell. A previous owner may have modified the turtle gear and/or the drive gear, due to the scarcity of turtles with the needed ratios.
 
ford38v8 said:
Don’t assume the turtle gear is correctly identified by the number on the shell. A previous owner may have modified the turtle gear and/or the drive gear, due to the scarcity of turtles with the needed ratios.
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I agree with Alan. You never know what has happened to your vehicle in the past 80 odd years.
 
Actually, you cant change the gear inside the turtle to another ratio. The axle that the gear rides on is located at different heights in the turtle to allow for different size gears. they need to match
 
;)
cas3 said:
Actually, you cant change the gear inside the turtle to another ratio. The axle that the gear rides on is located at different heights in the turtle to allow for different size gears. they need to match
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Actually, you can, by modifying the turtle shell itself. I believe there are Barners who have posted their method of doing exactly that.
And, in considering the original poster’s statement that his speedo registered half the actual speed, that just may be possible by removing every other tooth from the gear. I would think that any of those modifications would self destruct in short order, but then, this hobby does attract some folks who’d jump right on it.
 
If possible try another speedometer.You can put an 18 tooth gear in a 19 tooth turtle housing.I have done it several times.
 
I believe the difference in height is minimal, so as Deuce says, some slight variation is probably possible, I've not experimented with that. I am puzzled by the half speed reading. I find it hard to believe that the wrong turtle can change it that much, but, I usually try to get the correct turtle for the application. They are not hard to get
 
I to am puzzled by the 1/2 reading.BTW the 17 tooth gear & turtle housing is the most difficult to find.Had 1 in 55 yrs.
 
I checked the rear ratio and the turtle gear before I posted the question. My question is are there different drives for the driveshaft or did Ford use two different ratio speedometer. As far as all this talk about the turtle gear ratio, that would only change the speed a few miles not half.
 
I have seen pics of a 39 std speedo that went only to 70 MPH.I think they were for the big trucks.Yes there were different tooth count drive gears from 4-7 tooth.The 4 tooth was used with AA and BB trucks.I don't know if that would interchange with pass car and light trucks.
 
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I have cleaned and lubed speedo inputs by using a short piece of cable in an electric drill running in reverse. Full speed on the drill = about 3200RPM= about 70MPH on the speedo.
 
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