Speedometer gear
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Speedometer gear
I have an 88 F250 4x4 7.3 with a ZF 5 speed 3:55 gear ratio. I believe the transmission came out of a truck with 4:10 gears because the speedo is off. When I am going about 55mph I am really doing about 65 mph. Where can I find replacement gears? What tooth count do I need? I am running 235/85 R 16 tires. Thanks.
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From: Booneville, Missouri
Speedometer Gear
I have an 88 F250 4x4 7.3 with a ZF 5 speed 3:55 gear ratio. I believe the transmission came out of a truck with 4:10 gears because the speedo is off. When I am going about 55mph I am really doing about 65 mph. Where can I find replacement gears? What tooth count do I need? I am running 235/85 R 16 tires. Thanks.
I am in need of a gear also so I just replied to this thread so I can purchase a gear once a member answers this thread.
Take care and Godspeed.
Boon
Fleet Mechanic
Joined: Jan 2011
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From: 8600 ft in Colo
You need to pull the speedo cable out of the transfer case & count the teeth on the gear on the end of the cable. Then take the speed the speedo is showing and divide it by the speed you are actually going. Multiply that number by the number of teeth on the speedo gear, and that will tell you how many teeth you need.
For example, let's say your existing gear has 20 teeth. If I read you correctly, your speedo reads 55 when you're going 65. 55/65=0.846. 0.846x20=16.92, so you'd need a 17 tooth gear...
For example, let's say your existing gear has 20 teeth. If I read you correctly, your speedo reads 55 when you're going 65. 55/65=0.846. 0.846x20=16.92, so you'd need a 17 tooth gear...
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Joined: Jun 2013
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From: Booneville, Missouri
Speedometer Error
You need to pull the speedo cable out of the transfer case & count the teeth on the gear on the end of the cable. Then take the speed the speedo is showing and divide it by the speed you are actually going. Multiply that number by the number of teeth on the speedo gear, and that will tell you how many teeth you need.
For example, let's say your existing gear has 20 teeth. If I read you correctly, your speedo reads 55 when you're going 65. 55/65=0.846. 0.846x20=16.92, so you'd need a 17 tooth gear...
For example, let's say your existing gear has 20 teeth. If I read you correctly, your speedo reads 55 when you're going 65. 55/65=0.846. 0.846x20=16.92, so you'd need a 17 tooth gear...
My situation is just the opposite: If my speedo reads 65 for example my actual speed is 55 what formula would I use.
This is good stuff here.
Thank you
Boon
P.S. I have a 2WD.
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Indicated speed divided by actual speed times the number of teeth on the existing gear. For yours, assuming a 20-tooth gear again, that would be 65/55*20, or
1.182 * 20, or
23.6 -- round up to 24 teeth.
Now, don't go buy a 24-tooth gear just because some Joe Random on the Internet said to do so unless you count teeth on your existing gear and find out you actually do have a 20-tooth gear.
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Elder User
Joined: Jun 2013
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From: Booneville, Missouri
Speedometer Gear
Same formula, Boon. S(i)/S(a)*T == new tooth count, or
Indicated speed divided by actual speed times the number of teeth on the existing gear. For yours, assuming a 20-tooth gear again, that would be 65/55*20, or
1.182 * 20, or
23.6 -- round up to 24 teeth.
Now, don't go buy a 24-tooth gear just because some Joe Random on the Internet said to do so unless you count teeth on your existing gear and find out you actually do have a 20-tooth gear.
Indicated speed divided by actual speed times the number of teeth on the existing gear. For yours, assuming a 20-tooth gear again, that would be 65/55*20, or
1.182 * 20, or
23.6 -- round up to 24 teeth.
Now, don't go buy a 24-tooth gear just because some Joe Random on the Internet said to do so unless you count teeth on your existing gear and find out you actually do have a 20-tooth gear.
Thank you so much for the valuable information.
I am curious to see which gear is in my transmission.
Take care and have a great day!
Boon
Fleet Mechanic
Joined: Jan 2011
Posts: 1,709
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From: 8600 ft in Colo
Phy
Elder User
Joined: Jun 2013
Posts: 712
Likes: 0
From: Booneville, Missouri
Speedometer Gear.
www.summitracing.com also sells speedometer gears.
I can get a 20 tooth for $15.97. Just enter speedometer gears in the search box.
Take care
Boon
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