The Stovebolt Site Forums The Shop Area Driveline Speedometer gear in transmission

Took 4-speed transmission from 1950 3100 1/2 ton Chevy to transmission shop for rebuild. He said the transmission was missing the speedometer gear and that he would guess at placing one and then I would have to see how accurate it is when driving it.

Is there a standard size speedometer gear that should go on those transmissions?

Drive or driven gear?

Thanks. My understanding is that there is a small gear inside the transmission that drives the speedometer cable that is missing on this transmission. Shop explained that if they guess the wrong sized gear, the speedometer reading won't be accurate. Shop said that it is possible to connect something between to correct, but I wanted them to ideally put the right size gear inside from the start if that info is out there.

The drive-gear is on the transmission shaft, and the driven-gear screws into the transmission (with the speedometer cable going into the end of the driven-gear piece).

These gears are matched at the factory to the tires and to the rear end.

The probability of finding a matched-set of 1/2 ton gears in an SM-420 is low. If that transmission was in any other sized truck (or, was in a 1/2 ton with different diameter tires than on the present truck), the gears will not result in a good speedometer reading.

My suggestion is to find/use any set of drive/driven gears for an SM420 with enclosed drive-shaft-end. Then, do the measurement to order a ratio converter (The Speedometer Shop is one place that makes these). I have done this twice - it provides spot-on mileage (w/3.55 rear end and/or taller diameter radial tires).

You want the speedometer cable to turn 1001 rev per mile. Measure rolling circumference by making a chalk mark on the tire & floor, roll vehicle forward untill mark on tire again lined up w/ floor, make second mark on floor & measure the distance between the 2. Now for the math: 63,360" in a mile, devide by tire rolling circumference = tire rev / mile. Devide this by the axle ratio and you know propshaft rev / mile. The ratio between the drive gear(on the transmission output shaft) & driven gear(mates to speedo cable) will equal the ratio from propshaft rev / mile & 1001 (speedo cable rev / mile).

Incredibly helpful information.
I really appreciate it.

Maybe this one?

http://www.ebay.com/itm/NOS-1949-1957-Chevy-Speedometer-drive-Gear-Unit-3695417-Group-4-337-GM-NIB-/350723079436?fits=Year:1949|Make:Chevrolet&hash=item51a8b97d0c&item=350723079436&pt=Vintage_Car_Truck_Parts_Accessories&vxp=mtr

Part Number: 3695417; in Group Number: 4.337 (on box - not 4.332 as shown in eBay listing)

I think that part number is for a 3-speed transmission or Powerglide: 48-53 1/2 ton (standard transmission - 3-spd)

The 4-speed transmission has a different part number: 48-53 4-speed in 1/2 ton

We do a lot of driveline swapping be it engines, trans, or rear ends and have found the easiest for us is to use a GPS to get an accurate speed and then compare that to the speedometer reading. Say the Gps shows 55mph and the speedometer reads 65mph, that's about a 15.5% error so the DRIVEN gear neads slowed dowm 15.5%. On older or rare trans the easiest is to let some place like the Speedometer Shop make you up a correction unit. On popular trans a wide selection of both drive and driven gears can be found at dealers or places like TCI and The Transmission Shop. Google "speedometer gears" and there is a large offering. You can pick a set that gets you where you need to be. Most of our stuff is street or resto rod so we have been using the GPS speedometers now available. Transmission type, tire size, or rear gearing has no effect on a dead accurate reading so the guys that street drive during the week and race on weekends can switch out anything they want and not have to worry about how fast the're actually going when meeting John Law.

Found this. It's exactly what I'm looking at.

Tim, I'm calling the speedometer shop for a driven gear since I put 3.55 gears behind my sm420. Any help is much appreciated. Thanks

Hi Gentlemen,

I'm coming into this conversation late but am having speedometer issues too - my conclusion is that I was missing a part in the transmission too because my new speedometer cable does not turn when the truck is being driven (but will turn by hand when not connected to the transmission).

My question is, If I get one of these gears or drivers you mention, is that something I can put in relatively easily from in the cab with the floorboard out? (rather than needing to disassemble the transmission).

Here is a picture of what I see on my transmission connection if it helps. https://goo.gl/photos/GxJN1rrbF88B68t98

Thanks,

Doug

Hidiy, Doug-

You should be able to get to it from under the truck or in the cab, but what is the truck we're looking at? (year/make/trans/etc)

Easy, less than 600 seconds job on some of them... it is not a complete transmission disassembly is what I mean. On some transmissions, there is just a capscrew to remove to access the little gear with a confusing nomenclature.

Brad

Thanks Brad,

I have a '40 half-ton with a '53 or so 235. 4 speed transmission with a granny gear. (not sure what year).

Is the place to insert the gear the same point/hole that the speedometer line attaches to? (pictured here: https://goo.gl/photos/GxJN1rrbF88B68t98)

Thanks again,

Doug

Doug,

The piece in your last photo is the speedometer driven gear.

You can take that out to inspect its condition.

If all the teeth look good. You may have a bad (or, more likely, missing) speedometer drive gear. It is mounted on the forward yoke of the 4-speed u-joint assembly which resides inside the enclosed driveshaft's u-joint enclosure. You would have to disassemble the u-joint and drop/raise the drive shaft enclosure for removal of forward yoke and for inspection for the drive gear.