Speedometer

Tom F OHIO

Tom F OHIO

Well-known member
I installed a new speedometer cable and a 21 tooth gear. Took it out for a test drive and it is 10 mph under. Would a different gear fix this. Thanks for any help. Tom....
 
You need to match up your tire size with the correct speedo turtle number. There is a chart in the green book on this, or a search here would probably bring it up
 
cas3 said:
You need to match up your tire size with the correct speedo turtle number. There is a chart in the green book on this, or a search here would probably bring it up
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Thanks Cas, I will get my tire size and post it here so that somebody maybe can tell what I need. Thanks, Tom
 
you will need to calculate the percentage of error, not mph at a given speed. as i'm sure it is not 10mph under at all speeds.

i suggest testing at 10mph, 20mph, 30mph, etc. and with a little math it should be possible to come up with a percentage of error. from that, using the number of teeth on the "the gear" (sometimes called the "turtle") it should be possible to determine the number of teeth on it that will provide the most accuracy. and then the search for a new gear can begin.

in my case i calculate the percentage of error and then, when on the road, i calculate the actual speed in my head (using multiples of ten to simply things) if i find it necessary. but i rarely do. in general, i don't really care, as i usually drive along with the speed of other drivers as it's proven to be the safest and most courteous way to drive.

i've noticed that most of the modern cars i drive that have not been modified in some way have a speedometer that overestimates the speed somewhere between five and ten percent. i suspect for liability reasons.
 
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Huey, I checked it at different speeds and every time it was under by 10. My tire size is
P205/75R15. Not sure about the chart that was mentioned. Thanks for the info. Tom...
 
Tom F OHIO said:
Huey, I checked it at different speeds and every time it was under by 10. My tire size is
P205/75R15. Not sure about the chart that was mentioned. Thanks for the info. Tom...
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i may be wrong, so forgive me if so, but that's highly unlikely, i think.

for it is my understanding, that affixed to the end of the cable in the gauge itself is a magnet that turns in direct proportion to the mainshaft of the transmission and that in turn induces a magnetic field that affects the turning of the gauge's needle. and to prevent the needle from spinning as fast as the magnet is spinning, the needle is connected to a spring that only allows it to turn a fixed distance depending on two factors. one is the strength of the induced magnetic field created by the spinning magnet on the end of the speedometer cable, and a second factor based on the strength of the spring. the effect is proportional to the speed of the turning cable in all circumstances, i think.

using a tire size might be helpful but finding the percentage of error is easier in all respects, IMO.

for example if it turned out that the speedometer was registering ten percent too slow under all circumstances (i.e. 18mph instead of 20mph, or 27mph instead of 30mph or 36mph instead of 40mph, etc. then you would need a gear ("turtle") with 10 percent fewer teeth, so as to make the cable turn about 10 percent faster at all speeds. and so if the current gear ("turtle") has 22 teeth, one with 10 percent fewer teeth, IOW a 20 toothed gear would be what was needed.

BTW, as no car or model year was specified, i'm assuming the speedometer is associated with a ford manufactured between the mid 30's and mid 40's.
 
Assuming you were driving at 50 MPH when you are reading 10 MPH under, you are 20% off. Your 21-tooth gear is too fine, is not turning the cable enough as each tooth passes. A 17-tooth gear would turn 20% more, cable turns 20% faster.
 
Try a driven gear (the one at the end of the cable) that has fewer teeth. The fewer the teeth, the faster the speedo will go. There is a 19 tooth gear and a 20 tooth gear available. I see it in the CASCO catalog (Tbirds) which should work on a 52 Ford sedan if the sedan is stock or has a trans for a car prior to 1957.


The 19 tooth should get you a lot closer to actual


You might find the gears on ebay, although most of them are for 57 and later cars.
 
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Tom F OHIO said:
Thanks Cas, I will get my tire size and post it here so that somebody maybe can tell what I need. Thanks, Tom
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The Green Book table goes up to 1948

560650
 
Thanks for all the replies. I'd be lost without this site. I'm going to try a lower gear and see what that does for me. When I bought this car in May I didn't know very much about it, but I sure am learning. Thanks, Tom....
 
Shoebox Central list the 19 tooth gear for a non overdrive with 3:73 rear axle and the 21 for a overdrive. Supports what everyone has said about less teeth.
 
There is some incomplete or misleading information here. In a post-1948, the speedometer gear is related to the rear end ratio. It makes no difference if the vehicle has overdrive or not. The gear is on the output shaft.

To get the correct number of teeth gear, you must know the rear end ratio.

Ken
 
Also, your modern tire size will not calculate in the green bible chart, you'll need to know the diameter of the tire and the rear end ratio
 
I had a 21-tooth gear for the speedometer. I installed a 19-tooth gear in its place, and it is still 10 MPH off. When I'm going 35 its showing 25. Do you think my speedometer is bad. Pretty disappointed. Thanks, Tom...
 
i guess it's possible that whatever is being used to determine the "correct" speed is faulty. although if using a GPS app on a phone, that's not likely.

i think the change from 21 to 19 teeth is only about a 9.5% difference. meaning that, i think, if traveling at 30mph with a 21 tooth gear one should expect to see 27mph with a 19 tooth gear. unfortunately not much of a difference. and if your speedometer is like that on my '41 i doubt if i would be able to detect it. of course things get better as speed increases...
 
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