Paul

Please start over.

 Did the speedometer read correctly before the transmission change? ANSWER YES or NO.

The speedometer cable is connected on one end to .... The speedometer, The other end with the gear with the different number of teeth is connected to the OUTPUT SHAFT of the transmission. If you change the gear ratio of the REAR END the speed of your car will change with the same engine RPM.

So you will have to know if you have 7 teeth or 8 teeth on the transmission output shaft ( the thingy that the drive shaft connects to) and the axle ratio. Then get a chart and find the number of teeth on the speedometer gear to give you the right speed. Pete

It doesn't matter what the diameter of the gears is for a given tooth count. Say a 3.50:1 gear set has a 35 tooth ring gear and a 10 tooth pinion. It doesn't matter what diameter either of the gears is, the ratio will always be 3.50:1 because the pinion will always have 10 teeth and the ring gear will always have 35. When you change the ratio, you change the number of teeth which changes the speed of the pinion relative to the axles. The tooth count is all that's important for determining the ratio of any set of gears, be they speedometer gears or rear end gears.


Lawrenceville, GA

Pete, The speedometer was right before the transmission change.And I've been working on cars for 60 years so I know how rear gears change things.It's just a mystery to me how simply changing the number of teeth on the speedometer driven gear affects the RPM of the cable.

I just had a thought.Apparently the drive gear in the OD transmission is a slightly different diameter and tooth count than the drive gear in the FOM.It helps to think about these things early in the morning after 3 cups of coffee.

 

Paul,

Boonville,MO

Yes, FOM has 8 teeth and OD and STD transmissions has 7 teeth driving gear.


Tero from Nurmijarvi, Finland

56 Country Sedan, 61 F-100, 61 F-600, 55 F-620, 52 Mercury

http://www.dlc.fi/~teros

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Paul, your correct, thinking after 3 cups of coffee in the morning is much better that after 6 beers in the afternoon altho not nearly as creative or fun...

"The Master Cylinder"

Enjoying life at the beach in SOCAL

Folks the answer is what was the ratio of the rear end in the car the st/od was removed from. the speedometer gearing in the transmission was obviously set up for that.

To correct the speedometer you need to determine the speedometer error, a gps is great for this. Then apply this percentage to the tooth count to get the correct new gear. be aware the drive gear on the transmission output shaft may need to be changed due to compatability issues.

[quote][b]The Master Cylinder (5/19/2011) 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.



Not sure, but my '55 chassis parts catalog lists different speedo ass'y & dial face part#s (does anyone know if NOS speedos come with a dial face?). I assume your speedo became optimistic after installing the 150 mph dial face? If the % change mirrors the speedo error reported in all the '55 T-Bird car magazine road tests, then it's possible the speedo mechanisms were the same.


6 VOLTS/POS. GRD. NW INDIANA

I think you,re on right track with rear end thought, cant be exact but generally our cars are something like rear end ratios , Std 3 speed 3;7, O/D 2;89, FOM 3;2. so either way theres a difference that mixing trans with the rear ends will result in speedo error. At least thats how i see it.

  AussieBill            YYYY    Forever Y Block     YYYY

 Down Under, Australia

Sorry, didn't mean to hijack thread, just curious if anyone knew... My speedo was off anyway because I have a C-4 and 3:55's. Someday I am going to order a ratio adaptor http://flat-o.com/accessories/speedcable.htm from Flat-o, already have the two piece cable. Gene assures me we can get it close. Right now I just use my Tach.

"The Master Cylinder"

Enjoying life at the beach in SOCAL

This should help you figure out what gear you need-

http://www.sccoa.com/faq/speedgr.html

Ol'ford nutCentral Iowa

56 Vic w/292 & 4 spd.