speedometer gear

[57chevyman]

hey,
i have a new speedometer cable, and my stock speedometer is reading slower than actual speed. i have a local street with one of those electronic signs that tells you how fast youre going. the sign is 100% accurate. so when im going 30mph, my speedometer is only reading 20 mph.

the speedometer is also not going down to 0 when i stop... it goes down to 10mph and stays there

[SCHRUMGMC]

At least yours works, mine doesn't work at all

[OrrieG]

Transmission type?? Speedo is off because of tire size or possible speedo being gummed up. You can get different gears for the transmission to adjust the speed. Use the box at the top of the page with "67.." clicked using "changing speedometer gear" and a bunch of threads show up. Look for one that matches the transmission you have. I have stock 14 and low profile 15 inch tires and wheels for my Chevelle and there is a 7 mph difference at 70 I need to mentally adjust for when running the mags.

[SCHRUMGMC]

Orrie, where can I find a speedometer gear for my '59?
The one that goes in the transmission, there is one that
is for sale on Ebay but I am unsure that it is the correct
one

[57chevyman]

Originally Posted by OrrieG

Transmission type?? Speedo is off because of tire size or possible speedo being gummed up. You can get different gears for the transmission to adjust the speed. Use the box at the top of the page with "67.." clicked using "changing speedometer gear" and a bunch of threads show up. Look for one that matches the transmission you have. I have stock 14 and low profile 15 inch tires and wheels for my Chevelle and there is a 7 mph difference at 70 I need to mentally adjust for when running the mags.

i have a three speed manual. someone told me its a saginaw ?
i dont think the cable is gummed up.... its brand new.

is the speedo gear quick and easy to swap out? maybe i should get like 3 different ones and figure out which one makes the speedo accurate?

[57chevyman]

,,,,, if i count the teeth on my current speedo gear.....do i need a speedo gear with more teeth or less teeth if i need the speedo to read a higher speed than it does now?

[Nordic]

May be this link will help
http://www.tciauto.com/tc/speedometer-gear-calculator/

[OrrieG]

Here is what the speedo gear housing look like. The clip is bolted to the transmission housing and holds it in place. The gears have different number of teeth to adjust the cable rpm in relationship to the output shaft. Gears are available at GM BUT in one the the posts in one of the threads in the attached search said they were not the same diameter. I will let you read thru the posts, I scanned them and there are lots of links to speedo places.

http://www.google.com/custom?domains...D%3A1%3B&hl=en

[_Ogre]

Originally Posted by 57chevyman

hey,
i have a new speedometer cable, and my stock speedometer is reading slower than actual speed. i have a local street with one of those electronic signs that tells you how fast youre going. the sign is 100% accurate. so when im going 30mph, my speedometer is only reading 20 mph.

the speedometer is also not going down to 0 when i stop... it goes down to 10mph and stays there

first... before re-gearing the trans to make the speedo read right make sure the odometer reads right

get on the freeway with mile markers and do a 10 mile check
compare 10 miles on mile markers with 10 miles on your odometer and then get the percentage your off that way
use the speedo gear websites to get the odometer geared right

if the speedo is still off then get it calibrated
tbone is a member on this forum, he can calibrate speedos for a lot less than i could locally

the speedo not returning to zero indicates a problem in the speedo
this could be the needle hitting the glass or the back plate or a dead fly in the works
it could also be the needle just needs to be turned to zero, this also means that at 30 mph your needle would only go to 10 mph
i've had my speedo out too many times to count over the past year, it's not difficult to remove

i'd start by removing your speedo and freeing it up
then do the odometer test
then worry about calibration

there is nothing magic about the odometer it's all gearing, 1000 turns equals 1 mile on the odometer
that also equals 1000 rpm at 60 mph, this is where the magic enters the equation
magnets and eddy currents confuse this ogre

[mr48chev]

It's a lot easier to use a hand held GPS to check and calibrate your speedometer than either trying to do the mile post check or have someone in another vehicle pace you. If you don't own one or someone in the house doesn't own one you most likely have a friend who has one.

On the gear diameter thing. The diameter of the driven gear is matched to the diameter of the drive gear on the output shaft of the trans. Usually you can run 3 or 4 different driven gears with a specific drive gear. The link Nordic gave in post 7 is a good one. I usually go to this one http://www.partshp.com/speedometer_gears.htm I have found that it is very complete for most of our needs with trucks or hot rods.

The gears can usually be found though the local dealership parts counter or quite often at a transmission shop that has a good parts department.

On calibrating it, drive down the road and figure out exactly how far off it is at a set speed. Then you figure out how many teeth you need to change to get it into calibration. If it reads too fast you need a bigger driven gear to slow it down and if it reads too slow you need a smaller driven gear to speed it up.

Now to add to the confusion there also are ratio adapters that connect to the piece on the trans and the speedometer cable connects to them. They have small gears inside that can be switched around to calibrate the speedometer rather than taking the trans apart. They are used a lot of semi trucks and 4x4's where tire size changes are common and are much easier to work with. Not a bad thing to have if you are thinking about changing tire sizes.

[_Ogre]

good call mr48
there's a speedo app for android that has an odometer
i used the speedo app while my speedo was out for repair

[OrrieG]

My Garmin gps has a mode that show real time speed and distance.