Inliners International Forums General Discussion Drive Train Speedometer Drive Gears for SM 420

Anybody have any luck finding Speedometer Drive Gears for SM 420 Transmissions to go with 4.10 rear ends to speed up Speedometer after swapping from 5.14 and 4.56 ratios any body got part numbers or any information on this.????
Thanks ,Jesse Leroy

They should be out there- I have a '50 half ton which has a factory SM420 and 4.10 gears.

I am steady looking went thru about 20 or 30 4.56 and 5.17 rear centers before I found the 4.10 center section. Crawled thru lots of dirt and weeds. Almost wet myself when I fouind it! Transmision was gone already.Wonder if they are the same as later models or 3 speeds for that matter I will research it. Still hunting for goodys after all these years! Cant seem to have enough good stuff!
Take er easy greasy.

Leroy - where you been!?

Two options to correct speedo deviation when you change rear gears:
1)You can order a ratio adapter from a speedo shop (nearly all offer this) which is a gear set that will go in between the speedo and the tranny - usually on the tranny but it can go on the speedo head too
2) Pull the speedo and send it to a qualified shop for calibration based on your setup.

I chose the latter and sent my speedo to Scot Speedometer in Pontiac, MI where they dialed it in for the 4.10 rear gear swap I did on my 8 lugger. Now I'm installing an A833 OD and I am going to use option 1 for a ratio adapter because I can't stand the thought of another contortionist day under the dash of the '51 pulling the speedo AGAIN!

Dear Jesse;

Check with Tom's Truck Parts in Maimi,Ok., & see if he has something. The # is; 918) 542-3871 and there close to you.

Otherwise; you'll need to do what Rob sugusted.

Speedometer Shops are getting "few & far between" these days. There's Williamsons in 'south' too.

Good luck.

Rob,
Had tried the ratio adapter approach and found to the best of my Knowledge that Stewart Warner no longer makes them. They had them listed in the last catalog that I received from them but not in the newest catalog. Dug through all my stuff and found 5 or 6 different ones; modified one; and got it close for now. I think that we will all have to look into electronic speedometers sooner or later! Just rebuilt the electronic drive in my Cummins Dodge piece of cake.
I have been in hiding for awhile due to a thief that was stealing from me for awhile before I figured it out and laid out to catch him!
If The new "Texas Castle Law" had been in effect he would be buried instead of in jail.
I have one of those A 833 overdrive boxes that I have been eyeing also.The "Texas-Beast" is finally getting new wiring!(Cummins Powered 49 one ton Chevrolet Pickup)

Leroy, if that is the smaller sized speedo gear that GM used (and it should be) they came in sizes from 17T up to 24T. I usually found that each tooth made a difference of about 5 mph. When you are figuring out how many more or less teeth you need, don't forget that this is the DRIVEN gear, not the drive gear for the speedo cable. I can get you GM part numbers or source out the gears for you once you find out what you need.

I was digging everywhere hi and low for a certain driven gear for an early Muncie. I stumbled upon this place. They have gears and drive parts for almost anything and have ratio adapters to increase and decrease depending on what you need. Also a fair amount of tech info as well as transducers for the newer electronic based transmissions.

http://www.transmissioncenter.net/speedometer_calibration_______va.htm

I bought an adapter and service was quick.

Dear Mike;

Did you try 4speeds.com ??

John,

What I needed was a 20 tooth driven that mates with the 1.760 diameter steel drive gear. Nobody had anything less than a 22 tooth driven and nobody knew for certain if there ever was a 20 tooth gear made. I checked all of my local and on line trans parts sources and even some long shots like Corvette Central and several tri - five Chevy venders.

For the later Muncies and TH350 as well as the 3 speed all sychro manuals that use the 1.840 diameter nylon drive gear the choices are better, ranging from 17 to 22 teeth drivens. Nice of they'd work but they won't.

Oh the joys of working with vintage equipment!!!

My original SM 420 has 13 teeth on steel driven gear.This matches up to 5.14 or 5.17 rear ratio. According to my 1929 to 1955 Chevrolet parts manual they were available as sets of gears, from 11 tooth driven to 14 tooth driven; paired to a 4 tooth worm gear at the drive shaft yoke. They are not at all like the later Hydramatic 350 etc.Like I say got it close with a modified ratio adapter I built up myself, Im about 3 mph fast at 60 miles per hour and about 2.5 miles over on odometer in 100 miles travel. Can adjust speedometer internally to get the speed right and deduct 2.5 miles in a hundred to get fuel consumption correct enough!
Too poor to store buy a fix!