The Stovebolt Site Forums The Shop Area Driveline Speedometer drive gears

My 1952 3100 1/2 ton 4.11:1 differential pinion and ring gears wore out. Replaced them with 3.55:1 gear set. My truck has an original compound low 4 speed. Now I need speedometer gears that will up my speedo more or less 13%. Do any of you know of sources for speedometer gears for that 4 speed Transmission? Thank you.

Check online you may be able to find a adapter that will make your speedo read correct

Someone can correct me if I am wrong but the speedometer gears come as a set. One is pressed on to the u joint yoke and the other screws into the transmission case. You may have to change both with the SM420 ( I know you do for the predecessor 4 speed crash box tranny). That would be a kind of big job, but very doable. You would eventually find a set on Ebay.

It would be much easier to have an adapter made that just takes a minute to install. You can try some of the speedometer shops that advertise in Hemmings.
Kent

What size tires are you running? That will have a big impact on to what size speedometer gear you need. Do you have a local speedometer shop?

Also keep in mind that some speedometer shops can make an adapter based upon the difference between your truck's speedometer reading and that of a speedometer ap off of your phone. When I swapped my gears from 4.11 to 3.55, I told the shop what the speedo read at the same what my phone's speedometer ap (Speedbox) read. They produced an adapter which reflected the difference. Dead on!

My speedometer is way off. I changed the rear axle and it now has highway gears and the wheels were changed to to 15". In the 1970's Canada changed to metric speed limits and of course, the speedometer is in MPH. To avoid speeding tickets, I have to remember that in a 60 KmH zone my speedometer should read no more than 27 MPH.

Doing the conversion in my mind should help keep dementia in control. I hope. If it's not too late...

If the original poster is not interested in buying the ratio adapter from Jim Carter, here is a different company that sells a mechanical ratio adapter and an electronic (ERA) adapter.

My speedo is under by 20% throughout the speed range. I've done the math and keep the real speed in my head.

Some of us have a pretty good stash of ratio adapters, ask someone might have on.

I have one of the aforementioned speedometer ratio adapters but I have no idea what ratios it changes. It is made by Stewart-Warner with the following numbers: 668F S12 1 2020 X. If anyone knows how to decode these numbers it may be usable to somebody.
I have no use for it so if someone is interested you can PM me.
Fred

Way back when, I swapped in a Dana 60 with 3.54 gears, and the truck came with 4.57 gears. The speedometer read 65 but I was doing at 80. I used the tachometer to know what the speeds were, and created a chart for when I tried a different tire size, so I knew what the new rpms were reading at specific speeds. I also didn't know about speedometer adapters back then, so there is that...lol...

Look for a decimal point. Have 2 in front of me, .7333 (1 : 0.733) & 1.2020 (1 :1.20).

I've us a GPS speedo app on my phone. Drive 50 mph by digital speedo app, indicated speedometer reading above or below GPS X 2 = % needed read correct.

I can recommend Bobs speedometer shop, I took a speedo and tach from my Vette to him, he fixed me up while I waited.

Thanks BC, there is no decimal point but it's old. The 1: 1.20 is logical for the one I have.
Fred