Speedometer question
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Logistics Pro
Joined: Mar 2011
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From: Central Wisconsin
Speedometer question
So i went from having 4.10 gears in my F250 to 3.55 gears. I also put on 35 inch tires instead of the stock 30.5 size. how far off is my speedometer going to be? Im not entirely sure how to figure it out without driving it, and it doesnt run right now because the fuel pump went out. Thank you to anyone that helps!
Postmaster
Joined: Mar 2008
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From: Des Moines, Iowa
use this online calculator with the OEM equipment (ie:get the stock tire size off the door jam sticker) and then the current setup. that will tell you what drive and driven gears to use...
http://bronco.zenseeker.net/Speedo.htm
http://bronco.zenseeker.net/Speedo.htm
Fleet Mechanic
Joined: Jan 2011
Posts: 1,709
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From: 8600 ft in Colo
Calculating it out, it looks like your going 30.25% faster than the speedo indicates.
You'll need a speedo gear on the end of the cable with 30.25% fewer teeth on it than what's there now...
You'll need a speedo gear on the end of the cable with 30.25% fewer teeth on it than what's there now...
Posting Guru
Joined: Sep 2008
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So i went from having 4.10 gears in my F250 to 3.55 gears. I also put on 35 inch tires instead of the stock 30.5 size. how far off is my speedometer going to be? Im not entirely sure how to figure it out without driving it, and it doesnt run right now because the fuel pump went out. Thank you to anyone that helps!
I used this on my truck when I went to a larger tire.
FTE Legend
Joined: Jun 2006
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From: Ottawa, Ontario
It's probably safe to say your speedo is WAY off so it will need to be recalibrated. What year is the truck? '92+ have electronic odometers which indicates the presence of a PSOM which is programmable and since they use the sensor on the rear axle ring gear as a pickup the gear ratio doesn't affect anything only tire size does. If you have this system follow the link to see how to correct your speedo.. HOW TO RECALIBRATE THE SPEEDOMETER ON 92-96 FORD F-SERIES AND BRONCO .: Articles
If however you have a pre '92 truck the speedo is cable driven and recalibration is accomplished by replacing the driven cable on the trans/transfer case end of the cable. But to figure out what replacement gear to get you first have to know what color gear is on the cable and how far off your speedo is, and for that you need a GPS.
If however you have a pre '92 truck the speedo is cable driven and recalibration is accomplished by replacing the driven cable on the trans/transfer case end of the cable. But to figure out what replacement gear to get you first have to know what color gear is on the cable and how far off your speedo is, and for that you need a GPS.
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Postmaster
Joined: Mar 2008
Posts: 2,703
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From: Des Moines, Iowa
It's probably safe to say your speedo is WAY off so it will need to be recalibrated. What year is the truck? '92+ have electronic odometers which indicates the presence of a PSOM which is programmable and since they use the sensor on the rear axle ring gear as a pickup the gear ratio doesn't affect anything only tire size does. If you have this system follow the link to see how to correct your speedo.. HOW TO RECALIBRATE THE SPEEDOMETER ON 92-96 FORD F-SERIES AND BRONCO .: Articles
If however you have a pre '92 truck the speedo is cable driven and recalibration is accomplished by replacing the driven cable on the trans/transfer case end of the cable. But to figure out what replacement gear to get you first have to know what color gear is on the cable and how far off your speedo is, and for that you need a GPS.
To figure out the color of your gear that cable Paul is talking about... The cable will run to the tailshaft of your tranny or transfer case and there should be 1 bolt holding down a C-shaped clip. The C-shaped clip holds down the speedo driven gear. Remove bolt and clip and pull on the cable until the speedo gears pop out. Check the color of your gear (or count the teeth). I went to ford and bought 2 of each color so I can calibrate the speedo to my GPS by swapping the driven gear. Nice to have the extra gears just in case the tires change size in the future
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FTE Legend
Joined: Jun 2006
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From: Ottawa, Ontario
To figure out the color of your gear that cable Paul is talking about... The cable will run to the tailshaft of your tranny or transfer case and there should be 1 bolt holding down a C-shaped clip. The C-shaped clip holds down the speedo driven gear. Remove bolt and clip and pull on the cable until the speedo gears pop out. Check the color of your gear (or count the teeth). I went to ford and bought 2 of each color so I can calibrate the speedo to my GPS by swapping the driven gear. Nice to have the extra gears just in case the tires change size in the future
Ford Speedometer Gears for Automatic and Standard Transmission
On a related note I think I have to recalibrate the speedo in my truck again, I had to replace the speedo when the original died and the replacement seems to read high... though I haven't checked it against the GPS yet. That is strange I thought a speedo is a speedo but maybe they are slightly different depending upon how the truck is setup. My truck uses a cable driven speedo so if I get a chance to do this soon I'll snap some pics for future reference.
Postmaster
Joined: Mar 2008
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From: Des Moines, Iowa
On a related note I think I have to recalibrate the speedo in my truck again, I had to replace the speedo when the original died and the replacement seems to reads high... though I haven't checked it against the GPS yet. That is strange I thought a speedo is a speedo but maybe they are slightly different depending upon how the truck is setup. My truck uses a cable driven speedo so if I get a chance to do this soon I'll snap some pics for future reference.
PS: if you want to calculate the actual number of teeth needed by the drive gear, tire size, and axle ratio you can go here and get the calculation off the bottom of the page.
http://www.fordracingparts.com/download/charts/129.pdf
New User
Joined: Nov 2011
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Hello I need some help with this.... my truck is a 97 F-150 factory tires was 265/70R 17SL and currently is straight axle conversion with 5:13 gears sporting 35X12.50X15 my speedo is reading from my transfer case as I have 78 3/4 ton axles but speedo is electronic and uses a gear and is reading 25 mph fast and shuts down at 105 please help
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From: Ottawa, Ontario
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