The Stovebolt Site Forums The Shop Area Driveline Need different speedometer gear

After several months I finally found a machinist who could repair the messed up speedometer gear on my 1952 Chevy 3100. He used an 18 tooth plastic gear I had purchased that looked like the correct gear but the one piece plastic shaft that was part of it was too short. He cut the new gear off and pressed it onto my old shaft. My speedometer is now working but not very accurate. When it says I’m going 35mph I’m actually going 48, so it’s about 25% off. My thinking is i need the speedometer cable to spin 25% faster. I’m also thinking to do that I need a gear that has 25% fewer teeth. My current gear has 18 teeth so 25% is 4.5 teeth. If I bought another gear that had 14 teeth, would that get my speedometer close to being correct? Would it have a problem meshing with the gear inside my transmission that turns it?

You can buy adapters to compensate for that that thread into the tranny and then the cable threads into the adapter.

Speedometer ratio adapters

www.transmissioncenter.net

First thing you need to do is determine if your speedo inaccuracy is linear or not.

If 35 indicated is 48 actual,
is 25 indicated 34 actual?
is 30 indicated 41 actual?
is 40 indicated 55 actual?

If so, then a gear ratio change can fix the problem. Either put a different tooth count speedo gear into the transmission or buy an adapter.

If the actual speeds vs. indicated speeds are not linear as I posted above, changing the gear ratio will only "fix" the speedometer at one speed. You'll need to get someone to repair the speedometer first. Then decide if the gearing needs changing.

Also, I'm pretty sure your thoughts about changing to a lower number of teeth is wrong. You want to slow down the rotational speed of the cable. To do that you need to change to speedo cable gear with MORE teeth, not less.

Yup for an adapter. Way back then, I remember doing a lot of measuring etc before I was able to determine which adapter ratio was best for me, but now it is spot on. Now, using a GPS speedo to compare with your truck's speedometer will help determine what direction you need to go, and much easier. The link that Otto supplied above will have more details. Also for those deciding to install larger or smaller diameter tires, will also run into this issue and must plan to go the adapter direction.

BillyDon, a couple of years ago I made a GPS speedometer for my 1952 which looks pretty much like the original. This question has come up enough lately I'm considering making my next "project" a GPS speedometer that will fit right in and replace the original speedometer so the speedometer will be accurate no matter what. You can see what I made below. (I did add right/left turn signal dots, but those can be omitted if desired)

Jon, your GPS speedometer looks amazing! Since I’m pretty sure my speedometer is not the problem, I’ll probably go with an adapter. But it ever conks out, your solution would be very tempting.

Thanks BillyDon! It is sort of a blend of old/new but it is accurate and it has a re-settable odometer plus it is smooth and silent. The numeral fonts and the two-toned dial were one of the big challenges. If you need to make a new speedometer face decal or any of the individual gauge decals, the file for that is free and should be at the top of the Interiors section as a sticky file.

BillyDon, If you do go the adapter direction, using the cel phone GPS speedometer ap called Speedbox, makes it easy to compare your truck reading vs actual speed.

You are better off checking the odometer when looking for gear changes. The odometer is gear to gear, speedometer is gear to magnet drive. If the magnet drive is weak, the speedometer can be correct at one speed and off at another, usually the faster you go the farther off it gets.
If you are sure the speedometer is good, use 55 or 65 mph as your target speed if thats where you drive the most. Slower speeds are easier to guest-a-mate.

Just in case I decide to go with changing the speedometer gear, can someone confirm that Bill Hanlon is correct and my thinking is bass ackwards. If I’m traveling 40 mph but my speedometer is reading 30mph, I figured I would need to spin the cable 25% faster. To do that I would need a gear with 25% fewer teeth. Bill says I need to slow my cable down to increase the reading on my speedometer. I don’t understand how that works.

Have you confirmed how far it’s off ratio wise by the odometer and not the speedometer?

I drove a 13 mile route (according to the odometer on my Tacoma) and my odometer said 8 miles. According to my math, that’s too low by 38%. Since I now have a machinist who can inexpensively take one of those plastic gears and press it onto my shaft, I would rather do that than pay $150 for a ratio adapter. So I’m back to my question: if the current gear has 18 teeth, how many teeth do I need to correct the 38% I’m off?

Use this to get what gear your need for drive or driven

https://www.5speeds.com/calculators/SpeedoCalc.html

It has allowances for tire size. And rear end ratios

BillyDon,
Though you've already mentioned that you'd prefer to not go via an adapter, back around 2014 I went thru a similar issue as you, when I changed rear end gears, etc. Using a GPS speedometer ap, I was traveling about 20% faster than what my truck's speedometer was saying. I contacted Performance Automotive and Transmission Center in Bossier City, LA who said I needed a 20 speed up ratio adapter. Got it and installed it where the speedometer cable connects to the tranny and never looked back. Just something to consider.

Plus, with the ratio adapter you can install it yourself in under 10 minutes. You don't have to go into the transmission to fish out gears, etc.

Ok. You guys have convinced me to go with a ratio adapter. Thanks for all the feedback. These forums are an incredible source of information.

Message edited by adding the following paragraph:
The message I posted below was in response to BillyDon's message of 8/27/24. Unfortunately, I posted before reading subsequent posts that show that he did as JoeH suggested and found that the odometer was off by 38%. Sorry that I pulled the trigger too soon. I'll try to do better.



BillyDon:

My reply to your question "can someone confirm that Bill Hanlon is correct and my thinking is bass ackwards" got lost in the problems our forum has been having the last few days. I'll re-post.

Your thinking was correct. I was the bass ackwards one. You do need a speedo gear with less teeth. MAYBE.

I still recommend checking the speedo at various speeds, not just at the 35 mph indicated was actually 48 mph. If the problem is strictly linear, the proper ratio adapter will fix it. But if the problem is not linear, the ratio adapter will only fix it at one speed. The speedo on my '57 GMC read low at 30 mph, gradually got "right" at about 58-60 and then read low again at 75. An old-time speedometer shop in Houston did a clean-up and adjust on the speedo and had it within 2 mph from 30 to 80.

There were two suggestions made in this thread that you didn't follow up on. Either one will tell you if a ratio adapter will solve your problem.

My suggestion was to check the speedo at other speeds. If a ratio adapter will fix your problem, your speedo should read like this:

Indicated Actual
35 .........48 This is the only error you reported
25 .........34 Does your
30 .........41 speedo read
40 .........55 like this?

Or do like JoeH suggested and check the odometer for a proper reading. Start at a milepost on an Interstate highway. Record your odometer. Drive to and stop at the 10 miles away milepost. Subtract starting odometer reading from ending reading. Each 1/10th of a mile off of 10 miles is a one percent error in the gearing.

Or, buy a ratio adapter first and hope for the best.

just remember you going for odometer accuracy first. then adjust speedometer itself for accurate MPH reading.

hcb3200. I have a similar situation where the speedometer is accurate but the odometer lags. How do you adjust one without affecting the other?

The odometer is gear driven all the way to the numbers. so you make it accurate.
The speedo needle is driven by a gear to a magnetic driven spring. hence why you see it bounce every once in a while or lag
This part is adjustable on most speedometers.

Thus the reason lots of guys wonder why their milage is off when they fix their gears to a speedo that is out of Wack.

on the 55-59 series truck for example you slide a bracket to adjust as it "ages" or while cleaning and reassembly it needs adjusting.
some other years have a screw etc.

i'll track down the tech tip

The tech tip is here https://www.stovebolt.com/techtips/gauges/speedometer/index.html

if you look at where the spring is against the bracket on the back of the speedometer. That bracket will move you go in different directions to adjust tension. little bitty movements.

odometer gears all the way to numbers
speedo needle gears to a magnetic /spring driven needle.

Thanks for that info. Was not aware of such an adjustment

Heres how I fixed mine in my '37. The speedometer had set outside in the dash for 30 years so it was pretty crusty. I gave it a good cleaning with brake cleaner, then a couple of washes with WD-40 blowing it out carefully with an air hose till the WD-40 came out clean. I then oiled it lightly with sewing machine type oil. I have a hand held tachometer made for small engines, the type with a rubber cone on the end that reads off the end of a rotating shaft. I clamped a drill motor in the vise and set the rpm to 1000 rpm using the hand tachometer. GM used 1000 cable rpms to indicate 60 miles per hour. With the drill running at 1000 rpm my speedometer was reading slow, yet the odometer was clicking off 1 mile per minute just as it should. I pulled the needle off the gauge, I had it off while adding new decals, then set it back on the shaft but didn't push it down tight. I put it back on the drill running at 1000 rpm and carefully moved the needle to read 60 mph. I know it's not accurate at any speed but 60 mph, but its plenty close for my driving.
Knowing the speedometer and odometer were accurate at 60, it only took a 10 mile drive out and back to see how much I needed to adjust the driven and drive gears in the transmission. There are plenty of online calculators to use for the gear selections. Using highway markers, mine is still 2% slow with new tires, I can't get any closer due to the limited gear selection, mines is in between gears, either slightly to fast or slightly to slow. The adaptors can get right on the money. I have driven through plenty of the speed checker radar trailers here in the Kansas City area and have found speeds to match my speedometer most all the time, or as close as I can see when wizzing by them!

Just an update to my problem:
I just installed a ratio adapter with a 37% decrease. I took my truck for a spin and my speedometer now reads within a mile or two of the speedometer app on my phone at several different speeds from 20 mph to 45 mph. Thanks to everyone who responded to my post. You guys are the best.