GEM '62

Does anyone have a source for a ratio adaptor to correct a speedometer inaccuracy?

Crunch527

I talked to someone at Zips a few years ago who helped me figure out what color speedo gear to use when I changed gears but I imagine other vendors know how to figure this out...its a very common question.

For example, I changed rear gear ratios from 3:08s to 3:55s...subsequently years later went to a 5 speed and then went to a 4:11 and changed speedo gears yet again, in all instances, I was successful in "recalibrating" my speedo by changing the speedo gear.

Speedo gears are color coded which indicates the number of teeth. However, I cant remember the "rule of thumb" as to how to figure out which speedo gear.

The guy at Zips knew the tooth count "rule of thumb" and since I knew what speedo gear (color) I had, and I knew how much I was off, (+6mph) he was able to get me to the right speedo gear.

Maybe give Zips a call?

Frank

Powershift

Exactly what are you trying to achieve?? We may be able to help with gear colors and part numbers, etc. What did you change from original??

The Chevrolet Parts book has this info.....and it MAY be in some of the Chevrolet Service Manuals.

The GM ratio adapter I remember was used for running really low gears on the street, like 4:88 or 5:13 etc. Not certain that you need this....but let us know.

Larry

wmf62

did a google search for speedometer ratio adapter; there are lots to choose from; check ebay too.
Bill

Plasticman

There are some listed on Ebay:

http://www.ebay.com/sch/i.html?_from...adapter&_rdc=1

I got one from a Speedometer & AC repair shop in Orlando a few years back. Name was Ice Cold, but don't know if they still do that.

I think this was the company (their Facebook listing show "Speedometer Repairs").

https://www.facebook.com/IcecoldOrlando

1526 East Colonial Drive
Orlando, Florida 32803
Phone (407) 896-4661
Email ICAA4661@aol.com
Website http://www.IceColdAutoAirAndRepair.com


Plasticman

MasterDave

I took my car to a speedo calibration shop. They put an adapter on after checking for actual speed on their machine. Cost was around $150

GEM '62

My speedometer is 38% low so I can't correct by changing the driven gear, I would have to open up the transmission and also change the drive gear.
I found a supplier called Pact Speedometer World and ordered and adapter for $62.
Thanks for the responses.

Crunch527

apologize for missing the mark...getting educated on this one...

So how does a speedo end up 38% off??

Frank

GEM '62

The speedometer gears are matched to your differential ratio. If you change the differential ratio you will also need to change the speedometer gears. Depending on the amount of ratio change you may be able to just change the driven gear in the transmission, which is a 5 minute job, but if the difference is too great you will also need to change the drive gear which requires opening up the transmission. The ratio adaptor is a small gear box that is attached to the transmission between the transmission and speedometer cable. Much easier than pulling the transmission apart.
Tire size changes will also affect speedometer accuracy that can be corrected with gear changes or ratio adaptors.

Crunch527

Makes sense...thanks for taking the time to teach...much appreciated...

69ttop502

I just removed my tranny for this purpose. I am changing the drive gear as well since I found my car originally came with 4.56's. I don't see how you can fit a ratio adapter box between the tranny and cable, not much room there. I guess you can get another piece of cable and put it somewhere else under the car.

Bill

Plasticman

Some ratio adapters are right angle output, some are parallel (but offset) output, and some are direct inline output (with a box off to the side). Some will fit directly on the speedo output off the trans (depending on the shifter rods), but agreed it is safer to run a short cable to the adapter, and then the speedo cable.

Scroll down the Ebay listing below to see the various outputs:

http://www.ebay.com/sch/i.html?_from...adapter&_rdc=1

Plasticman

Avispa

Whichever option you choose, make sure the selection of speedo driven/drive gear and adapter (if used) results in a correct odometer calibration before you deal with the speedometer. This is easy enough to do using mile markers on a highway that has them. If you're off by only a couple of 10ths of a mile in 20 measured miles, your odometer will be off by about 1%. This will be the case whether or not you maintain constant speed, or even stop completely during the odometer check run; the odometer movement corresponds directly to the number or turns of the tires, which in turn is directly related to the distance the car has moved. There is no relationship to the speed at which the movement takes place.

Once the odometer set up is taken care of, then you can get the speedo calibrated if necessary so that the MPH reading will be correct for the RPM of the speedo cable. If the odometer calibration is correct, a correctly calibrated speedo will then show the correct car speed within the limits of accuracy of the speedo gauge itself because the speedo cable rotation speed will correctly correspond to the car speed.

GEM '62

I received my ratio adaptor today and it works great. There was some question about having room enough to install the unit, but that was not an issue. I also have a pulse generator for my cruse control that has to attach to the adaptor and there was still enough room for everything. Should have bought one years ago.

Bigdick

Does anyone know the colors of speedo gears in 1989 c4 with 700R4 transmission and 259 diff gears.

GUSTO14

You may want to post this question over in the C4 Tech/Performance forum...
https://www.corvetteforum.com/forums...erformance-48/

Good luck and welcome to the Corvette Forum... GUSTO

vettebuyer6369

Please start a new thread in the C4 section.