speedometer calculation
#1
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.
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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.
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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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#8
Join Date: Jul 2006
Location: La Mesa (East San Diego)
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"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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Join Date: Apr 2002
Location: Littleton, New Hampshire
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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.
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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
#13
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