Speedometer fix
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Speedometer fix
79 f150 2wd 300 3 speed( w/ unsync 1st gear) on the floor
The speedometer is about 20mph off and anything hwy speed and above. The needle jumps and is all over the place. As far as I know I run the correct tire size but I will check tomorrow. What kind of an ordeal am I looking at? I assume it's a cable. Any advice or info is much appreciated. Thank you
The speedometer is about 20mph off and anything hwy speed and above. The needle jumps and is all over the place. As far as I know I run the correct tire size but I will check tomorrow. What kind of an ordeal am I looking at? I assume it's a cable. Any advice or info is much appreciated. Thank you
Fleet Mechanic
Joined: Sep 2014
Posts: 1,626
Likes: 78
From: NE Texas
The needle jumping is probably the cable, as has been stated. Being off 20mph is a different kettle of fish, and would indicate that something has been changed since the original speedo/speedo driven gear were installed.
When you fix the cable and the needle is more stable, try going on the highway at an indicated 60mph and see if you are passing the mile markers at 1 minute intervals. If so, your speedo is indicating OK.
If not, you may have a case where tire size or differential gearing may have been changed and you will need to determine the final gear ratio you actually have now. (The speedo driven gear is the intermediary gear between the speedo cable and the transmission. Its size/number of teeth determines how many times your speedo cable turns in comparison to the driveshaft turns, so if you have one say for very tall gears like 2.75, and actually have very low 4:1 gears, your speedo would read wildly off.) They make boatloads of speedo driven gears so you should be able to get the correct one, and there are numerous posts on how to replace it. Could be that at some point someone just swapped any old driven gear in to fix a dead speedo and didn't verify it was the right size, too.
When you fix the cable and the needle is more stable, try going on the highway at an indicated 60mph and see if you are passing the mile markers at 1 minute intervals. If so, your speedo is indicating OK.
If not, you may have a case where tire size or differential gearing may have been changed and you will need to determine the final gear ratio you actually have now. (The speedo driven gear is the intermediary gear between the speedo cable and the transmission. Its size/number of teeth determines how many times your speedo cable turns in comparison to the driveshaft turns, so if you have one say for very tall gears like 2.75, and actually have very low 4:1 gears, your speedo would read wildly off.) They make boatloads of speedo driven gears so you should be able to get the correct one, and there are numerous posts on how to replace it. Could be that at some point someone just swapped any old driven gear in to fix a dead speedo and didn't verify it was the right size, too.
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