Speedometer off by about 10mph
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From: Gray, TN
Speedometer off by about 10mph
The speedometer in my '77 F250 2wd reads about 10mph faster than the actual speed. It was originally a 400 powered truck. I bought it with a '73 360 and 4 speed allready installed by a previous owner. I assume the truck the motor and trans came from had a different rear end ratio. What part do I need to change to correct this problem, and where can I buy it?
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The plastic speedo gear at the end of the speedo cable
http://www.tciauto.com/Products/Tech...eedo_gears.asp
Gear ratio and tire size affect speedo registering
https://www.ford-trucks.com/calculat...trument_error/
http://www.tciauto.com/Products/Tech...eedo_gears.asp
Gear ratio and tire size affect speedo registering
https://www.ford-trucks.com/calculat...trument_error/
Last edited by Mil1ion; Oct 12, 2007 at 07:31 PM.
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From: Simi Valley, CA
The speedo driven gears at the end of the cable where it fits into the transmission are still available from Ford. Adding one tooth will slow the speedo down, and solve the problem. Gears are available from 15 thru 21 teeth. You will need to know how many teeth your current gear has, before buying another. You will need to count the teeth of your driven gear, then ask for the next tooth count up.
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Originally Posted by MintFord
The speedometer in my '77 F250 2wd reads about 10mph faster than the actual speed. It was originally a 400 powered truck. I bought it with a '73 360 and 4 speed allready installed by a previous owner. I assume the truck the motor and trans came from had a different rear end ratio. What part do I need to change to correct this problem, and where can I buy it?
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From: Gray, TN
Thanks for the help guys. Once I fix this, I'll have to get used to NOT subtracting 10mph from my speed :-)
On my first drive on public roads I traveled what I thought was the speed limit. I noticed a long line of cars forming behind me. Driving in the 55 zone on the 4 lane at what I thought was about 62, I noticed that everyone was blowing past me like I was standing still. If I do what reads about 10 mph over what I would normally do in my other vehicles, i drive among traffic in my usual style of passing many cars and letting those in a big hurry pass me. It also resulted in the correct travel time to arrive at work which was approx 25 minutes away.
Originally Posted by 78f250supercab
I was wondering how did realize your speedo was off
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From: Simi Valley, CA
Originally Posted by MintFord
Thanks for the help guys. Once I fix this, I'll have to get used to NOT subtracting 10mph from my speed :-)
On my first drive on public roads I traveled what I thought was the speed limit. I noticed a long line of cars forming behind me. Driving in the 55 zone on the 4 lane at what I thought was about 62, I noticed that everyone was blowing past me like I was standing still. If I do what reads about 10 mph over what I would normally do in my other vehicles, i drive among traffic in my usual style of passing many cars and letting those in a big hurry pass me. It also resulted in the correct travel time to arrive at work which was approx 25 minutes away.
On my first drive on public roads I traveled what I thought was the speed limit. I noticed a long line of cars forming behind me. Driving in the 55 zone on the 4 lane at what I thought was about 62, I noticed that everyone was blowing past me like I was standing still. If I do what reads about 10 mph over what I would normally do in my other vehicles, i drive among traffic in my usual style of passing many cars and letting those in a big hurry pass me. It also resulted in the correct travel time to arrive at work which was approx 25 minutes away.
If you are a member of AAA, they have a traveling speedo calibration truck that visits cities in your area. You should have your speedo calibrated correctly by experts.
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From: Gadsden Purchase
http://www.panix.com/~alvinj/file12/SpeedTable.jpg
Scanned off the back of a railroad "time table". :)
Alvin in AZ (retired signal ape)
Scanned off the back of a railroad "time table". :)
Alvin in AZ (retired signal ape)
Last edited by Alvin in AZ; Oct 14, 2007 at 12:26 PM.
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