I have an approximate 10 MPH speedometer error at 60MPH (shows 70 MPH)    Do I need to replace the speedometer driven gear with one that has more teeth or less teeth?  Thanks

More teeth (?).  
I went through all this over 25 yrs. ago, so It’s a blur now. The T-Bird speedo was notoriously optimistic, and too bad BTW there was no smart-phone GPS back then when I had to calculate/include changed tire diameters also.  The rarity of orig. speedo gears, which side of the gear options to choose (correct ratio usually falls somewhere between available tooth #s), which speed range accuracy should be most important (mechanics of the 50s speedo usually means accuracy not proportional across the board), etc., etc.  Testing tachometer accuracy should likely come first (I used a non-variable drill rated at a specific speed….which was probably rather naive to trust).
 Reminds me why my grade school teacher claimed I would actually need algebra some day 😁.

6 VOLTS/POS. GRD. NW INDIANA

Alan, you'll want a driven gear with more teeth to slow down the speedometer. What year is your car, and which transmission do you have? 57 Fordomatics use the same driven gears and cables as the C4. 55-56 use a different gear, cable and an angle drive. I think the standard Fordomatic gear for cars equipped with a 3.31 axle ratio has 19 teeth. If your car is a 57, you'll find a wider selection of gears from which to use. If your car has a manual transmission, hopefully someone else will chime in. 

An easy place to get driven gears is ebay.  Wide selection


54 Victoria 312;  48 Ford Conv 302, 56 Bird 312
Forever Ford
Midland Park, NJ

I have a '54 Tudor Sedan Police car with 4.27 heavy duty station wagon rear axle, 3 speed O.D. transmission and the special 120 MPH police speedometer.  The '52 Shop Manual calls for a 22 tooth driven gear for this rear end but this would be for the standard 110 MPH speedometer.  The police speedometer has the numbers placed in different positions on the dial.  I have a 22 tooth gear in there now.  I read somewhere that the police cars used a 23 tooth gear but these are not available and impossible to find.  I think that I may just be stuck with the speedometer error.

I think you're right. If you have an accurate tach, you can use an online calculator to determine what your rpm should be at a given speed. Depending on your location, you shouldn't have anything to worry about when moving with the flow of traffic. You don't need a speedometer to tell you you're going faster/slower than everybody else. 

Alan, I may possibly have what you need. I too went through this dance several times and one of those times was with a '54 overdrive trans. I have several of those type driven gears here somewhere. I'll find them and let you know what I have.

64F100 57FAIRLANE500

Alan, i have 3 gears with tooth count 17, 18 and 19. Probably not what you need but you are welcome to them if you can use them. Looking through all this stuff I did find a lot of R10 overdrive trans parts though. Good luck with your interesting project...............

64F100 57FAIRLANE500

I did a quick look on google for a 23 tooth gear and found lots of them.  CJ Pony parts is one vendor.  Checked ebay, a lot there too.





54 Victoria 312;  48 Ford Conv 302, 56 Bird 312
Forever Ford
Midland Park, NJ

Yes, but I think that those 23 tooth speedo gears are the later 1957 and up style not the earlier style.  If I am mistaken Paul, please send me a link.  Alan