Speedometer Ratio adapter - The 1947 - Present Chevrolet & GMC Truck Message Board Network

AASmedic · 01-30-2018, 02:25 AM

I have a discrepancy between my speedometer and the actual speed because I changed out to bigger tires. I know that I can install a speedo gear ratio adapter to correct the discrepancy. It is off about 10% (Reads 60 when Im only doing 55). The question is finding out the correct ratio. I have a kit that I purchased recently that I can use to build the needed adapter and it came with many different gears but unfortunately the instructions are vague or non-existent. Any help would be greatly appreciated.

thanks,
kevin

**painterljp** · 01-30-2018, 08:22 AM

We downloaded a GPS speedometer on our phone set it in the windshield/dash and changed one gear at a time til we got it spot on. After you put one you will know more or less the directions you'll need to go. Or call the guys at trans star. They are alot of help.

B. W. · 01-30-2018, 11:24 AM

Never been inside one, I'm assuming there are three gears - drive, idler, driven? If so, are the idlers all the same tooth count? If they are you can calculate output by dividing the drive gear by the driven gear. If the idlers have different tooth counts you can still calculate, I'd have to look it up, I do all my math on my fingers!

MARKDTN · 01-30-2018, 11:36 AM

Can't you just change the driven gear (and maybe the drive) in the transmission? That would seem a lot cheaper than a ratio adapter-and less moving parts to wear.

sick472 · 01-30-2018, 02:15 PM

There are websites that offer "speedometer gear calculators". You'll have to know...

number of teeth on the drive gear (in tranny),
tire diameter, and
rearend ratio.

If you have found your truck to be inbetween somewhere then the ratio adaptor may be your only hope.

rockyrivermark · 01-31-2018, 08:40 AM

I bought a set of the plastic speedo gears of eBay.
Tried a few until I got one that read correctly. Simple to change and only takes 5 min to swap out.
Download free phone app for mph and test that way.
Mark

AASmedic · 01-31-2018, 12:05 PM

Thanks for all of your input folks. I will look into solving this discrepancy with the info provided.

geezer#99 · 01-31-2018, 12:20 PM

I had one of those multi adjustable adaptors once. Never got it close enough. Tried the gear change in the tranny and that was a pain in the butt.
Since then I’ve used adaptors from these guys. They have it all. Gears, calculators etc.

https://transmissioncenter.net/

01-30-2018, 02:25 AM	#1
AASmedic Registered User Join Date: Dec 2015 Location: Albuquerque Posts: 773	Speedometer Ratio adapter I have a discrepancy between my speedometer and the actual speed because I changed out to bigger tires. I know that I can install a speedo gear ratio adapter to correct the discrepancy. It is off about 10% (Reads 60 when Im only doing 55). The question is finding out the correct ratio. I have a kit that I purchased recently that I can use to build the needed adapter and it came with many different gears but unfortunately the instructions are vague or non-existent. Any help would be greatly appreciated. thanks, kevin __________________ My Paypal address is: http://paypal.me/KFerguson66 72 K20 Cheyenne Super My "re-build" thread http://67-72chevytrucks.com/vboard/s...?t=710221&goto Thread for my Reproduction Jack Bags http://67-72chevytrucks.com/vboard/s...d.php?t=715455

01-30-2018, 08:22 AM	#2
painterljp Senior Member Join Date: Nov 2014 Location: Albuquerque NM Posts: 901	Re: Speedometer Ratio adapter We downloaded a GPS speedometer on our phone set it in the windshield/dash and changed one gear at a time til we got it spot on. After you put one you will know more or less the directions you'll need to go. Or call the guys at trans star. They are alot of help. Last edited by painterljp; 01-30-2018 at 08:23 AM. Reason: Spelling

01-30-2018, 11:24 AM	#3
B. W. Registered User Join Date: Nov 2014 Location: Bigfork, Montana Posts: 1,137	Re: Speedometer Ratio adapter Never been inside one, I'm assuming there are three gears - drive, idler, driven? If so, are the idlers all the same tooth count? If they are you can calculate output by dividing the drive gear by the driven gear. If the idlers have different tooth counts you can still calculate, I'd have to look it up, I do all my math on my fingers!

01-30-2018, 11:36 AM	#4
MARKDTN Registered User Join Date: Jan 2011 Location: Chattanooga, TN Posts: 2,154	Re: Speedometer Ratio adapter Can't you just change the driven gear (and maybe the drive) in the transmission? That would seem a lot cheaper than a ratio adapter-and less moving parts to wear. __________________ '83 K20-TPI '73 C10 '79 C10-ex-diesel(SOLD) '07 Tahoe(Son driving) '14 Suburban-DD '71 C10-current project

01-30-2018, 02:15 PM	#5
sick472 Registered User Join Date: Jul 2015 Location: Sedalia Mo. Posts: 1,131	Re: Speedometer Ratio adapter There are websites that offer "speedometer gear calculators". You'll have to know... number of teeth on the drive gear (in tranny), tire diameter, and rearend ratio. If you have found your truck to be inbetween somewhere then the ratio adaptor may be your only hope. __________________ He who is without oil shall throw the first rod. Compressions 8.7:1 1972 C10 1976 C10 (parts truck) 1985 K20

01-31-2018, 08:40 AM	#6
rockyrivermark Registered User Join Date: Apr 2011 Location: Fairview Park, Ohio Posts: 1,031	Re: Speedometer Ratio adapter I bought a set of the plastic speedo gears of eBay. Tried a few until I got one that read correctly. Simple to change and only takes 5 min to swap out. Download free phone app for mph and test that way. Mark __________________ My build page http://67-72chevytrucks.com/vboard/s...d.php?t=764662 My cabinet building site http://www.cbcabinets.com

01-31-2018, 12:05 PM	#7
AASmedic Registered User Join Date: Dec 2015 Location: Albuquerque Posts: 773	Re: Speedometer Ratio adapter Thanks for all of your input folks. I will look into solving this discrepancy with the info provided. __________________ My Paypal address is: http://paypal.me/KFerguson66 72 K20 Cheyenne Super My "re-build" thread http://67-72chevytrucks.com/vboard/s...?t=710221&goto Thread for my Reproduction Jack Bags http://67-72chevytrucks.com/vboard/s...d.php?t=715455

01-31-2018, 12:20 PM	#8
geezer#99 Registered User Join Date: Aug 2009 Location: Bowser Posts: 13,667	Re: Speedometer Ratio adapter I had one of those multi adjustable adaptors once. Never got it close enough. Tried the gear change in the tranny and that was a pain in the butt. Since then I’ve used adaptors from these guys. They have it all. Gears, calculators etc. https://transmissioncenter.net/



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