|
By PWH42 - 14 Years Ago
|
|
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?
|
|
By Pete 55Tbird - 14 Years Ago
|
|
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
|
|
By Nick Brann - 14 Years Ago
|
|
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
|
|
By Genuinerod - 14 Years Ago
|
|
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.
|
|
By PWH42 - 14 Years Ago
|
|
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.
|
|
By DANIEL TINDER - 14 Years Ago
|
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.
|
|
By The Master Cylinder - 14 Years Ago
|
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. 
|
|
By charliemccraney - 14 Years Ago
|
|
PWH42 (5/19/2011)
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
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.
|
|
By PWH42 - 14 Years Ago
|
|
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.
|
|
By flathead_jr - 14 Years Ago
|
|
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.
|
|
By Pete 55Tbird - 14 Years Ago
|
|
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
|
|
By charliemccraney - 14 Years Ago
|
|
PWH42 (5/19/2011)
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.
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.
|
|
By PWH42 - 14 Years Ago
|
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.
|
|
By Teros292 - 14 Years Ago
|
|
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
Yes, FOM has 8 teeth and OD and STD transmissions has 7 teeth driving gear.
|
|
By The Master Cylinder - 14 Years Ago
|
|
PWH42 (5/20/2011)
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, 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...
|
|
By Don Woodruff - 14 Years Ago
|
|
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.
|
|
By DANIEL TINDER - 14 Years Ago
|
[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.
|
|
By aussiebill - 14 Years Ago
|
Don Woodruff (5/20/2011)
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. 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. 
|
|
By The Master Cylinder - 14 Years Ago
|
|
DANIEL TINDER (5/20/2011)
[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.
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.
|
|
By Ol'ford nut - 14 Years Ago
|
|
This should help you figure out what gear you need- http://www.sccoa.com/faq/speedgr.html
|
|
By PWH42 - 14 Years Ago
|
|
I've already figured out what I need to correct the speedometer.All I need is a 17 tooth gear which is what Ford would have used in a car with a 3 speed or OD transmission and a 3.23 rear gear,which is what I have.Only problem is that they are scarce as hen's teeth.Evidently very few 56 Fords were ordered with straight transmissions and 3.23 gears.I'll just have to live with a 20% speedometer error.
|
|
By PF Arcand - 14 Years Ago
|
|
Paul: unfortunately this won't help you.. it's unlikely that Ford ever supplied O.D. transmission cars with a 3.23 axle. That gearing is very high for that application, so finding that speedo gear isn't likely an option.
|
|
By PWH42 - 14 Years Ago
|
Paul........I know that would be a rare factory setup,but they did offer it as an option.The parts manual shows a 17 tooth gear for it.
For most uses,the 3.23 gear and straight transmission is not a very good combination,but it is working well for me.
|
|
By Ted - 14 Years Ago
|
|
Paul. You might try advertising for a 17 tooth gear for the ’56 cable in the classifieds. There may be some readers that are overlooking this thread.
|
|
By Nick Brann - 14 Years Ago
|
|
Hi Paul, I've had good luck for hard-to-find Ford parts with a company in Ohio. They are Green Sales, 2172 Seymour Avenue, Cincinnati, OH 45237. Phone number (I hope it's still good) was 513-731-3304. If you have a part number that will help, but if not they can usually look it up for you. Good luck, Nick Brann - K.C., MO
|
|
By PWH42 - 14 Years Ago
|
|
Thanks Nick,I'll give them a try tomorrow.
|
|
By PWH42 - 14 Years Ago
|
Gene in Kentucky,that sent me a PM about a speedometer gear.........I tried to reply to you,but it wouldn't go because you didn't provide your user name.Send me another PM with your email and I'll get back to you.
Thanks
|