Speedo running fast
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Speedo running fast
my 84 f250s speedo is running 5 to 10 mph faster than my actual speed ive tried tried tightening the cable on the t case and it didnt help what else can i do other than replace.
Lead Driver
Joined: Apr 2007
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From: Houston
I dont see how tightening the cable would change the speed the speedometer cable spins at. That would only be the speedometer gear, but we are talking transfer cases and I am not that big on four wheel drive so maybe its something new to me.
Posting Guru
Joined: Apr 2011
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From: Orlando area
I expect someone put bigger tires on the truck. The bigger tires travel further for each revolution (compared to stock tires) and therefore the speedo is off. Only a speedometer (or rear axle) gear change will fix this.
Unless this problem just started for you and it was working correctly before - if so I'm baffled.
Unless this problem just started for you and it was working correctly before - if so I'm baffled.
Ford Parts Specialist
Joined: Nov 2006
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From: Simi Valley, CA
The nylon speedometer DRIVEN gear at the end of the cable is available with 16 thru 21 teeth.
Whatever number of teeth the present gear has (except if it has 21 teeth), adding a tooth will slow the speedo down between 7 to 10 MPH
For example, if the present gear has 19 teeth, you want a 20 tooth gear.
The same nylon tooth DRIVEN gears have been used in myriad cars/trucks since 1960, are available from Ford and probably auto parts stores as well.
Whatever number of teeth the present gear has (except if it has 21 teeth), adding a tooth will slow the speedo down between 7 to 10 MPH
For example, if the present gear has 19 teeth, you want a 20 tooth gear.
The same nylon tooth DRIVEN gears have been used in myriad cars/trucks since 1960, are available from Ford and probably auto parts stores as well.
Lead Driver
Joined: Apr 2007
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From: Houston
Wouldn't running fast be caused by a smaller tire?
I went with a taller tire and it slowed my speedometer down as the tire moved a greater distance per revolution resulting in the speedometer gear to spin slower.
For me I checked mine via gps speedometer when my speedometer was reading 40 mph I was really going 42-43 mph. It his going from a stock tire to a 31x10.50-15 tire. If you look most of these tires are really not 31" tall but range from 29" to 30".
I went with a taller tire and it slowed my speedometer down as the tire moved a greater distance per revolution resulting in the speedometer gear to spin slower.
For me I checked mine via gps speedometer when my speedometer was reading 40 mph I was really going 42-43 mph. It his going from a stock tire to a 31x10.50-15 tire. If you look most of these tires are really not 31" tall but range from 29" to 30".
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Joined: Jun 2014
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The speedometer works based on the amount of revolutions made. Since the taller tire, by nature, has a larger circumference, one full turn of the tire yields a greater distance moved. A smaller tire yields a lesser distanced moved. So if the speedo reads faster than your actual speed, either smaller than stock tires are on your truck, or something is on the fritz
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Posting Guru
Joined: Apr 2011
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From: Orlando area
Usually the complaint is that the truck is going faster than the speedo says it's going after someone swaps to larger tires. I guess I just read the problem backwards based on what I "expected" was the complaint.
But the fix is the same. Either install stock diameter tires, change the differential gearing, or change the speedometer cable gear.
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From: Sheffield, AL
If the speedometer is actually working, but at the incorrect speed, then the speedometer cable gear is not bad... it has the wrong number of teeth for the current set-up.
If the gear was bad, either the speedometer would not work or it might be jerky.
Try using the tire size calculator [see attached link].
http://www.sizemytires.com/calculators/speedometer
If the gear was bad, either the speedometer would not work or it might be jerky.
Try using the tire size calculator [see attached link].
http://www.sizemytires.com/calculators/speedometer
Last edited by 1986F150six; Sep 29, 2014 at 10:35 AM. Reason: Added link
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the speedo used to work fine up until here recently. Now it was acting up a while back where id slow down and the needle would drop to 0 and when i would be getting down on it it would suddenly jump up to 75 and i know i wasnt getting up to 75 that fast
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BigBlockF350
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Mar 30, 2013 04:47 PM