Speedometer 5 mph off - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**BIG ORANGE** · 06-12-2022, 03:26 PM

71/C10 350/350 trans has gray gear on the cable, what color gear do I need to change to?
Speedometer reads 50 mph when going 55 mph
Thanxs

**Getter-Done** · 06-12-2022, 03:43 PM

There are calculators online.
You put in tire size, axle ratio and drive teeth.

Scroll down in the links below and both have calculators.
The second one explains the tire stuff better.

Link: https://transmissioncenter.net/shop/...ar-calculator/

Link: https://www.tciauto.com/speedometer-gear-calculator

**kwmech** · 06-12-2022, 06:52 PM

It is not off 5 mph, it is off 10%. So you need to speed up the cable by that amount

tim_mc · 06-12-2022, 07:08 PM

According to my notes, each tooth on the driven gear on the speedo cable changes the ratio by roughly 5%. After swapping to a 700R4 with 3.42 gears, my speedo reading was about 15% higher than actual, so I needed a driven gear with 3 more teeth to reduce the speedo reading. My driven gear was also gray (22 teeth). Check my findings, but seems like you might need a driven gear with 2 less teeth to increase your MPH reading (5MPH/55MPH=~9%). The OEM GM 20-tooth gear is blue, 21-tooth is red.

**Getter-Done** · 06-12-2022, 07:46 PM

There is also these Adapters.

https://speedometercablesusa.com/gear_box_adapters.html

**BIG ORANGE** · 06-12-2022, 08:33 PM

Thanxs guys,that's just what I needed..... the 20 tooth gear should fix my issue.

**Getter-Done** · 06-12-2022, 10:35 PM

Keep us Posted on results.
This will help the next guy

72SB · 06-13-2022, 10:01 AM

FWIW, same issue with me. I had a 21 tooth in my spare parts which got it closer. Bought the Blue 20 tooth but no work. Seemed like the drive gear was not engaging with the 20 tooth. IDK what drive gear my 3 speed Saginaw has (8??). 20 tooth should work (GM blue gear) but for some reason for me it didn't. Ideally your trans is happy with a 20

Accelo · 06-13-2022, 10:40 AM

Maybe the diameter sizes will help with understanding the speedometer issues.

Here is an example of the challenges;

If you measure the diameter of the gear, a new one is 0.81". It is designed to be used with only a 1.84" diameter drive gear.
You installed a green 22T driven gear. A new green 22T gear has a diameter of 0.87". This gear is designed to be used with a 1.74" diameter drive gear.

If drive gear in your transmission is 1.74" diameter. You cannot use the smaller diameter driven gears in the current transmission configuration. The gears do not mesh (teeth do not touch).

green 22T driven gear (0.87" diameter)---- 1.74" diameter drive gear red 21 T driven gear (0.81" diameter)---- 1.84" diameter drive gear

Also, if you try the other way (0.87" driven, 1.84" drive), the plastic driven gear will be destroyed after a short while.

Exactly why this is so complicated. To make this worse you can have the same drive gear (on the transmission output shaft) with the same tooth count in two different diameters.
GM made is easier in later years but that's a different story/post.

I have included a picture that indicates how to count the teeth on the drive gear by viewing it on the side. This is an eight tooth gear.
Cheers

**HO455** · 06-13-2022, 01:35 PM

Good explanation.

purple gas · 06-14-2022, 05:18 PM

why does there have to be math?

**Getter-Done** · 06-14-2022, 06:06 PM

Quote:

Originally Posted by purple gas

why does there have to be math?

**Getter-Done** · 06-14-2022, 06:11 PM

The link I sent above is for an External option.
There was (years ago) A box with gears you could change out,
That connected to the Transmission, and you hooked up the speedometer cable on the side of it.

That is probably what they offer? IDK

LNP · 06-14-2022, 07:06 PM

In this artcile from Chevy Hardcore they say each tooth is 3mph.

https://www.chevyhardcore.com/tech-s...to-correct-it/

Accelo · 06-14-2022, 08:55 PM

There was (years ago) A box with gears you could change out".

You still can purchase the conversion boxes.
You will spend $125 to $150 to get one configured correctly.
You can get them for 1/2 that on Ebay but finding the correct ratio isn't easy and you have to do the math either way.

tutone · 06-14-2022, 09:07 PM

Quote:

Originally Posted by purple gas

why does there have to be math?

Don't you remember your middle school teacher telling you that you would use math the rest of your life?

67C10Step · 06-15-2022, 10:40 AM

I've been watching this thread with keen interest as my speedo is about 10% off as well. When I had the gear and everything out last winter I am almost certain it was the gray one and I have a blue one on order now.

Oh yeah, as I told my wife once: "Your whole world is bounded by math. With the correct equation, I can predict almost anything." But, I am an egg head engineer so it's a day at the office for me.

**BIG ORANGE** · 06-17-2022, 04:53 PM

Well I looked through my parts drawers and found a lite beige colored gear with 19 teeth, I installed it and added a lil tranny fluid cause of the mess I made but any how, we left and went on vacation so I haven't got to drive it yet. Will tell yall how it goes as soon as I get back home.lol

06-12-2022, 03:26 PM	#1
BIG ORANGE Senior Member Join Date: Oct 2001 Location: Lancaster S.C. Posts: 4,744	Speedometer 5 mph off 71/C10 350/350 trans has gray gear on the cable, what color gear do I need to change to? Speedometer reads 50 mph when going 55 mph Thanxs __________________ Things happen for a reason!

06-12-2022, 03:43 PM	#2
Getter-Done Senior Member Join Date: Dec 2009 Location: TN. Posts: 7,727	Re: Speedometer 5 mph off There are calculators online. You put in tire size, axle ratio and drive teeth. Scroll down in the links below and both have calculators. The second one explains the tire stuff better. Link: https://transmissioncenter.net/shop/...ar-calculator/ Link: https://www.tciauto.com/speedometer-gear-calculator __________________ ________________________________________________________________________________________ ____________________________________________ 84 Chevy K-20 63 Impala (my high school car) http://67-72chevytrucks.com/vboard/s...Crew Cab Build

06-12-2022, 06:52 PM	#3
kwmech Senior Member Join Date: Mar 2003 Location: Colfax-California Posts: 8,391	Re: Speedometer 5 mph off It is not off 5 mph, it is off 10%. So you need to speed up the cable by that amount

06-12-2022, 07:08 PM	#4
tim_mc Registered User Join Date: Jan 2019 Location: St Peters, MO Posts: 434	Re: Speedometer 5 mph off According to my notes, each tooth on the driven gear on the speedo cable changes the ratio by roughly 5%. After swapping to a 700R4 with 3.42 gears, my speedo reading was about 15% higher than actual, so I needed a driven gear with 3 more teeth to reduce the speedo reading. My driven gear was also gray (22 teeth). Check my findings, but seems like you might need a driven gear with 2 less teeth to increase your MPH reading (5MPH/55MPH=~9%). The OEM GM 20-tooth gear is blue, 21-tooth is red. __________________ 1967 C20, 350 SBC w/Comp roller cam, 700R4 w/3.42 posi, PS, 4-wheel PDB, Old Air AC & GM Tilt column.

06-12-2022, 07:46 PM	#5
Getter-Done Senior Member Join Date: Dec 2009 Location: TN. Posts: 7,727	Re: Speedometer 5 mph off There is also these Adapters. https://speedometercablesusa.com/gear_box_adapters.html __________________ ________________________________________________________________________________________ ____________________________________________ 84 Chevy K-20 63 Impala (my high school car) http://67-72chevytrucks.com/vboard/s...Crew Cab Build

06-12-2022, 08:33 PM	#6
BIG ORANGE Senior Member Join Date: Oct 2001 Location: Lancaster S.C. Posts: 4,744	Re: Speedometer 5 mph off Thanxs guys,that's just what I needed..... the 20 tooth gear should fix my issue. __________________ Things happen for a reason!

06-12-2022, 10:35 PM	#7
Getter-Done Senior Member Join Date: Dec 2009 Location: TN. Posts: 7,727	Re: Speedometer 5 mph off Keep us Posted on results. This will help the next guy __________________ ________________________________________________________________________________________ ____________________________________________ 84 Chevy K-20 63 Impala (my high school car) http://67-72chevytrucks.com/vboard/s...Crew Cab Build

06-13-2022, 10:01 AM	#8
72SB Registered User Join Date: Apr 2022 Location: Moorpark, CA Posts: 701	Re: Speedometer 5 mph off FWIW, same issue with me. I had a 21 tooth in my spare parts which got it closer. Bought the Blue 20 tooth but no work. Seemed like the drive gear was not engaging with the 20 tooth. IDK what drive gear my 3 speed Saginaw has (8??). 20 tooth should work (GM blue gear) but for some reason for me it didn't. Ideally your trans is happy with a 20

06-13-2022, 10:40 AM	#9
Accelo Registered User Join Date: Mar 2015 Location: washington Posts: 2,162	Re: Speedometer 5 mph off Maybe the diameter sizes will help with understanding the speedometer issues. Here is an example of the challenges; If you measure the diameter of the gear, a new one is 0.81". It is designed to be used with only a 1.84" diameter drive gear. You installed a green 22T driven gear. A new green 22T gear has a diameter of 0.87". This gear is designed to be used with a 1.74" diameter drive gear. If drive gear in your transmission is 1.74" diameter. You cannot use the smaller diameter driven gears in the current transmission configuration. The gears do not mesh (teeth do not touch). green 22T driven gear (0.87" diameter)---- 1.74" diameter drive gear red 21 T driven gear (0.81" diameter)---- 1.84" diameter drive gear Also, if you try the other way (0.87" driven, 1.84" drive), the plastic driven gear will be destroyed after a short while. Exactly why this is so complicated. To make this worse you can have the same drive gear (on the transmission output shaft) with the same tooth count in two different diameters. GM made is easier in later years but that's a different story/post. I have included a picture that indicates how to count the teeth on the drive gear by viewing it on the side. This is an eight tooth gear. Cheers Attached Images Last edited by Accelo; 06-13-2022 at 10:46 AM.

06-13-2022, 01:35 PM	#10
HO455 Post Whore Join Date: Feb 2016 Location: Portland Oregon Posts: 10,807	Re: Speedometer 5 mph off Good explanation. __________________ Thanks to Bob and Jeanie and everyone else at Superior Performance for all their great help. RIP Bob Parks. 1967 Burban the WMB,1991 S(stink)-10 Blazer,1969 GTO, 1970 Javelin, 1952 F2 Ford 4X4, 29 Model A, 72 Firebird. 85 Alfa Romeo If it breaks I didn't want it in the first place The WMB repair thread http://67-72chevytrucks.com/vboard/s...d.php?t=698377

06-14-2022, 05:18 PM	#11
purple gas Registered User Join Date: Oct 2004 Location: saskatoon, planet earth Posts: 680	Re: Speedometer 5 mph off why does there have to be math? __________________ I got a bench seat baby, you don't have to sit over there.

06-14-2022, 06:11 PM	#13
Getter-Done Senior Member Join Date: Dec 2009 Location: TN. Posts: 7,727	Re: Speedometer 5 mph off The link I sent above is for an External option. There was (years ago) A box with gears you could change out, That connected to the Transmission, and you hooked up the speedometer cable on the side of it. That is probably what they offer? IDK __________________ ________________________________________________________________________________________ ____________________________________________ 84 Chevy K-20 63 Impala (my high school car) http://67-72chevytrucks.com/vboard/s...Crew Cab Build

06-14-2022, 07:06 PM	#14
LNP Registered User Join Date: Mar 2014 Location: Pekin, Illinois Posts: 333	Re: Speedometer 5 mph off In this artcile from Chevy Hardcore they say each tooth is 3mph. https://www.chevyhardcore.com/tech-s...to-correct-it/ __________________ Harley 1969 Chevy C-10 Long Bed, 350cid, 700R4

06-14-2022, 08:55 PM	#15
Accelo Registered User Join Date: Mar 2015 Location: washington Posts: 2,162	Re: Speedometer 5 mph off There was (years ago) A box with gears you could change out". You still can purchase the conversion boxes. You will spend $125 to $150 to get one configured correctly. You can get them for 1/2 that on Ebay but finding the correct ratio isn't easy and you have to do the math either way. Attached Images

06-15-2022, 10:40 AM	#17
67C10Step Registered User Join Date: Jul 2019 Location: Beebe, AR Posts: 2,407	Re: Speedometer 5 mph off I've been watching this thread with keen interest as my speedo is about 10% off as well. When I had the gear and everything out last winter I am almost certain it was the gray one and I have a blue one on order now. Oh yeah, as I told my wife once: "Your whole world is bounded by math. With the correct equation, I can predict almost anything." But, I am an egg head engineer so it's a day at the office for me. __________________ 1967 C10 1980 Jeep CJ5 2020 Toyota 4Runner 2020 Toyota CH-R 2020 Acura MDX



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06-17-2022, 04:53 PM	#18
BIG ORANGE Senior Member Join Date: Oct 2001 Location: Lancaster S.C. Posts: 4,744	Re: Speedometer 5 mph off Well I looked through my parts drawers and found a lite beige colored gear with 19 teeth, I installed it and added a lil tranny fluid cause of the mess I made but any how, we left and went on vacation so I haven't got to drive it yet. Will tell yall how it goes as soon as I get back home.lol __________________ Things happen for a reason!

Register or Log In To remove these advertisements.