speedometer calculation
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Postmaster
Joined: Apr 2004
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From: Santa Clarita
speedometer calculation
Since I've changed my wheels and tires and gear ratio in the rearend. My speeedometer seems to read 10 mph too slow. I have a C4 transmission and I know it's easy to change the driven gear to calibrate your speedometer. Has anyone done this? What is the rule of thumb as far as number of teeth to make the speedometer read correctly. I know with the 3.5 gears, it read pretty close to accurate? With the 2.75 gearing, I'm about 10 mph too slow. Any help is appreciated. In other words my speedo reads 50 but I'm doing 60 mph.
Fleet Owner
Joined: Jul 2004
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From: NM
I used to have the part numbers for C4 speedo gears; I think there are only 3 or 4 choices. They are color-coded. It seems to me that the more teeth, the faster the speedo would read for a given speed. NumberDummy will probably be able to set you up!
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Postmaster
Joined: Apr 2004
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From: Santa Clarita
Bob, if you pull out your speedo cable from the transmission, you'll notice a gear on the end. It's held with a C clip. Remove the clip and replace the gear with one with either more teeth or fewer teeth. That will adjust and calibrate your speedometer. It just a matter of which gear with how many teeth you need to put on????? You can buy the gears on epay, they run about $10 each plus shipping.
Fleet Owner
Joined: Jul 2004
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From: NM
Believe it or not, they were in stock when I went to my local Ford dealer and (ten years ago) were $6 apiece or so. They are apparently common to many different trannies (the one in my F-1 looks the same).
Roast em' if you got 'em
Joined: Mar 2005
Posts: 22,013
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From: Rio Rancho, NM
Somewhere online there is a chart. I was doing research about 6 month ago and found it (actually there are several)
My speedo is about 15mph fast (its on my todo list.....I'm waiting until I change diff gears)
I'll bet a search for C4 speedo gears should turn up some sources
Bobby
My speedo is about 15mph fast (its on my todo list.....I'm waiting until I change diff gears)
I'll bet a search for C4 speedo gears should turn up some sources
Bobby
Laughing Gas
Joined: Jun 2007
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From: Richland, WA.
I had a 79 Trans Am that needed the gears changed. I went to GM dealer told them my axle ratio and the gears I had, Tailshaft and cable.They ordered the correct one. I'm sure Ford can do anything GM can do, only better. Don't forget about transducers ( I think they are called). They go between speedo cable and trans to fine tune speedo. The TA had one. Kurt G. Y-blocks rule.
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Senior User
Joined: Jul 2006
Posts: 448
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From: La Mesa (East San Diego)
"But Officer, I couldn't possibly have been speeding I changed the speedometer gear myself." 
I have done it and it's easy. It's as described above And, I have taken vehicles to speedo shops and had them fine tune it with those little inline gear box thingys Kurt is calling transducers. Works great. You could probably take it to a speedo shop and get the gear changed and if need be the transducer to fine tune, get it tested and a calibration card for less than $40 or so.
Also, if you have a tach in your vehicle and know the wheel role out (exact circumference), You can use the gear/transmission calculator on the site to tell you exactly how much the speedo should be off at any given RPM.
I have done it and it's easy. It's as described above And, I have taken vehicles to speedo shops and had them fine tune it with those little inline gear box thingys Kurt is calling transducers. Works great. You could probably take it to a speedo shop and get the gear changed and if need be the transducer to fine tune, get it tested and a calibration card for less than $40 or so.
Also, if you have a tach in your vehicle and know the wheel role out (exact circumference), You can use the gear/transmission calculator on the site to tell you exactly how much the speedo should be off at any given RPM.
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Fleet Owner
Joined: Jul 2004
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From: NM
So Imlowr, if you are reading 50 when you're going 60, you need a gear with 60/50 = 1.2X as many teeth as you have now. If you have a 24 tooth, you need a 28 or 29 (closest). If you are too far off, you'll need a converter piece like Dan said, or have to the speedo recalibrated..
Lead Driver
Joined: Apr 2002
Posts: 8,813
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From: Littleton, New Hampshire
I've been through this a few times. Generally, one tooth is worth about 5 MPH. If the speedometer is too slow by 10 MPH you need a gear wth two less teeth than what you have now to speed it up. If no such gear exists then you would have to get your speedometer recalibrated or go with some kind of converter. My truck is running a 2.26 rear gear with a TH350 transmission and there isn't a gear with fewer teeth available so I'm running about 6 MPH slow using a gear with the fewest teeth available. 
Posting Guru
Joined: Aug 2000
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From: S.C.
I just did this change a couple years go I went from a 2:75 to 3:55
. If I'm not mistaken I was running 16 teeth with the 2:75 rear end. I google speedometer gear calculator and found a calculator try this one Speedometer Gear Computation. I went to a local Ford dealer and brought two different sets for about $6 each. There is another web site that had the colors to match the different teeth
. If I'm not mistaken I was running 16 teeth with the 2:75 rear end. I google speedometer gear calculator and found a calculator try this one Speedometer Gear Computation. I went to a local Ford dealer and brought two different sets for about $6 each. There is another web site that had the colors to match the different teeth
Posting Guru
Joined: Oct 2003
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From: Adams,CO.
Ed, check out page 1 on the link. These guys are so awesome IMO. There will be no guess work, or measure. They can calc for you on gears,tire circ,speed. I just follow a pal in a car that is accurate at 60 MPH for a mile then call these guys and 3-4 days later I have my stuff. It is real simple. I think I used Part # 82 on 1 of my C-4 trannies. It is listed half way down the page. I think it's a pretty cool site overall and I hope it helps you out. Jon
Speedometer Gear, Speedometer Gears, Ford, GM, Dodge 700R4
Speedometer Gear, Speedometer Gears, Ford, GM, Dodge 700R4
Lead Driver
Joined: Apr 2002
Posts: 8,813
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From: Littleton, New Hampshire
More teeth on the driven gear slows down the speedometer and less teeth speeds it up. I went from 15 teeth to 11 on my truck to get it as close as possible...