I have a question that I should know the answer to,but I don't.I changed my car from FOM to a 3 speed overdrive and retained the original 3.23 rear end,so now the speedometer is way slow.Am I right in thinking that I need to change the 19 tooth gear for a 21 or 22 tooth gear?

 

Paul,

Boonville,MO

Paul

The speedometer "sees" the RPM of the outputshaft of the transmission so if you did not change the axle ratio ( still 3.23) then no. The gear should remain the same unless a different year transmission is being used with a different number of DRIVEN GEAR TEETH, then who knows? Pete

Hi Paul,  I assume when you say your speedo is too slow, that it reads slower than your actual speed.  If that is correct, then you need a gear with fewer teeth to make the cable turn faster.  Try a 17 or 18.  You can do the math, if the speedometer is 10% slow then you will probably need to go down 2 teeth from your present 19 tooth.  If I've read your message wrong, then you'll need to go the other way.  Good luck, Nick Brann - K.C., MO 

According to Ford parts & accessories text catalog, for rear ends with 3.22-3.31, it should have 17 tooth gear for conventional and overdrive transmissions.

Thanks for the help,guys.I'm still not sure in my mind(what's left of it)why changing the number of teeth affects the speed of the cable,unless the gears are different lengths and consequently ride at a different point on the drive gear.Oh well,another of lifes mysteries that I will probably never understand. Does any one know of a place where a 17 tooth gear would be available.All my vendor catalogs only show 19,20,21 and 22 tooth gears.

 

Paul,

Boonville,MO

Paul,



If you had planned switching to radial tires down the road (if not done already), the change of aspect ratio (diameter?) should be included in your math. Also, how accurate was your speedo before the trans swap (T-Bird speedos for example, were notoriously optimistic)? Has it been rebuilt & precision calibrated? Even so, antique style speedos seldom read accurate across their total range. You may have to decide whether city or interstate speed should be most correct. Plus (if your luck is like mine), you will likely find that available gear tooth #s fall over or under what is truly accurate, and you will have to choose which side to err on.

6 VOLTS/POS. GRD. NW INDIANA

Daniel, You brought up something that I always wondered about. Did Ford actually have two different Speedometers in ’55, one for the Car and one for the T-Bird or did they just change the face.



I put a T-Bird Speedo face on my Ranchwagon’s Speedo just to “Freak People Out”. It’s funny to watch people when they notice a 150 MPH speedo in a Stationwagon.


"The Master Cylinder"

Enjoying life at the beach in SOCAL

It's a gear ratio like any other on the car. Say a 3.50:1 rear gear, the drive shaft turns 3.5 times for every 1 turn of the axles. Switch to a 3.20:1 one rear gear and the drive shaft now turns 3.2 times for every 1 turn of the axle; the drive shaft turns slower than it does with a 3.50:1 gear. By changing the speedometer cable gear, you change the ratio, which changes the speed of the cable.


Lawrenceville, GA

But,when you change the rear gear ratio,you're changing both gears,including changing the diameter of the gears,not just the number of teeth on the gears.That's what changes the output speed of the axles.In the case of the speedometer gear,all that's changing is the number of teeth on the driven gear.One thing I don't know is whether all the different speedometer gears are the same diameter.

 

Paul,

Boonville,MO

My 52 to 54 Ford repair manual tell what gears to use with which ratios. There were different gears on the input shafts as well, 7 or 8 teeth I think. I actually like the FOM transmissions anyways. They are a 3 speed, you just have to shift from L to Dr to get the extra gear, and that's how I drive mine anyways.