x0000rgw

I want to re-calibrate my speedometer in my 61 vette.
Does anyone know the cable RPM to show a certain speed?
I would like to set the speed that is showing to 50 or 60 MPH.
What RPM of the cable would show this?
Thanks,
Ray

x0000rgw

From what I have found on the internet, the speedometer
should read 60 mph when the cable is turning at 1000 rpm.
If anyone knows more, please let me know. I am taking the
speedometer out today.

Ray

92GTA

I thought you just have to change the little plastic gears in the tail housing for whatever rear end ratio you have? GM even has charts for figuring out which color gears you need for a given ratio...

x0000rgw

I don't think you have read my post. I have the correct gear in
the tail shaft. The problem is that the speedometer does not show
the correct speed. You adjust this by setting the resistance spring
inside the speedometer.
when the speedometer shows 69 mph, I am actually doing 73 mph.
Thats what the highway patrol man said with his radar.
Ray

DZAUTO

Ray,
The FIRST thing that needs to be done, is to determine if the ODOMETER is correctly indicating the distance traveled-----------------NOT if the speedometer is indicating the correct speed!
To do this, drive the car a known distance (for example, 10mi between mile markers on a section of Interstate hiway).
Once the CORRECT plastic gear is in the tranny for the odometer reading, THEN the speedo can be adjusted.
The speedo is spun by a magnet inside the speedo head. THIS CAN BE VARIABLE AND CAN BE ADJUSTED. BUUUUUUUUUUT, the correct odometer reading is DIRECTLY related to the CORRECT gear installed in the tail housing of the tranny--------------it CANNOT be adjusted any other way!!! So, before you start screwing with an adjustment to the speedo, get the right gear for the odometer reading installed in the tranny first!
VERY PROBABLY, if your speedo is in good condition, then once the odometer is indicating the correct distance traveled, then the speedo will likely be indicating the correct speed.

To INCREASE cable speed, install a gear in the tranny with FEWER teeth and vice versa to reduce the cable speed.

jim lockwood

Yes, 1000 cable RPM should indicate exactly 60 MPH

Jim

x0000rgw

Thanks Jim,
I don't know how this question got so confusing. I am sure that
I have the right gears in the tail shaft. Am 8 tooth dirive gear, a
20 tooth driven gear and a 3.36 rear ring and pinion. I assume that
this will generate the proper RPM for the speedometer cable. Since
the odometer is a direct drive off of the cable, it should be fairly
correct with 27 inch tires. I just need to tweek the adjustment of
the indicated speed. This is magnetically driven by the speed of the cable.
Thanks for the help. You had the only sensible answer.
Ray

AZDoug

Yes.

Speedo shops actually move the car a short distance (possibly on rollers, also) and count the number of turns the cable makes to tell them if the trans gear works well with the tire diameter and rear end ratio. They will then change the gear, or make a ratio adapter to make cable turns match odometer reading.

The ten mile method works pretty good, as it can tell you recorded vs actual to within say 2%. You really need a passenger to start counting mile markers (and write starting odo value down) as you need to know EXACTLY where the 1/10ths wheel is in relation to its readout window when the first marker starts and the moment the ten mile is finished. A 50 mile drive works better yet and is more forgiving of being exact on position of the 1/10th wheel in relation to the window.

I know my odo/speedo reads about 14% low, but I do know my tach at 3000 RPM is exactly 75 MPH in top gear, verified by dyno and GPS, so every 200 RPM change is 5 MPH.

Doug

wmf62

once upon a time, in a land far far away (i believe it was California...), i had a speedometer calibrated by a speedometer shop. i watched him do it and it was done by adjusting the magnetism in the head, not by tweaking the spring.

Bill

JohnZ

You have the correct pieces for your axle and tires; finer calibration is a function of gauss/degauss of the magnets in the speed cup, not the tension spring. Most speedometer shops have the equipment to do this (with the speedo out of the car).

x0000rgw

John,
Thanks for the info on the spring. Guess I will try to find a shop
here in Dallas to adjust it. I went through this about 20 years ago
with a shop in Richardson,TX. It worked for about 50 miles and
the odometer locked up. I think I will get it "fixed" again if I can
find a shop that works on this old stuff.
Ray

Avispa

I'm with DZ - be sure the speedo cable rotation is correct by checking the odometer against known mileage before messing with the speedometer. You might be contemplating a lot of work for nothing. 1 tooth off on the speedo driven gear is about a 4-5 percent error on the indicated speed (and mileage). That'll cover going 73 when the speedo reads 69 for sure.

x0000rgw

Hello Avispa,

First of all, I have the correct gears, (Drive gear on tail shaft, driven
plastic gear in the housing)
Second, the odometer has nothing to do with the indicated speed
on the speedometer. The idea of re-magnatizing the spinning magnets
is garbage. There is an adjustment in the spring that controls the
needle that indicates the speed. That is what it is there to do. I have the parts ordered to fix the odometer. I will post how to do the
repairs when I am done. I will re-calibrate the speedometer with a
short piece of cable and a drill that runs at 800 RPM. At
800 RPM the speedometer should show 48 MPH.
If my work fails, then I will get a 64 Impala speedometer and adapt it.
Ray

wmf62

i respectfully disagree.
Bill

67*427

Me too.....

1snake

Well, everyone is kind of right AND kind of wrong. Here's a bit of info. I found about speedometers.

Recalibrating a speedometer can be done by manipulating the hairspring, the permanent magnet or both. Generally, the strength of the magnetic field is the easiest variable to change. This requires a powerful electromagnet, which can be used to adjust the strength of the permanent magnet in the speedometer until the needle matches the input from the rotating drive cable.

0Willcox Corvette

Ok.. I'm not going to say who is right or wrong... just explain how I do it....

The main adjustment in all speedometers is the speed cup tension spring! You can't see it in the picture below but it is there on the speed cup. This spring is attached to a lever on the speed cup retainer and is used to dial in the speedometer by applying more tension to the speed cup. The speed cup is driven by the magnet as show in the picture below.

While calibrating speedometers... It is possible to have a magnet that has lost some of it's zip.. in this case we zap them to bring them back up. This is usually not the case and in most instances the speedo can be dialed in by tweaking the spring either lighter for less tension or tighter for more. A sign that the magnet needs a charge is a speedo that can not be dialed in. A good example of this would be one that can be correct at 30 and off at 60 and no matter where you put the spring it just won't dial in to range..

There are other issues when rebuilding speedometers such as one having the replacement 6458122 head with the flat magnet. (has less drive). These things were a nightmare for me since the magnet won't have the same pulling ability as the stock magnet. The springs in stock speedometers have more tension and the flat magnet can't seem to pull it good enough to work. So, to use this magnet you must also change the spring to accommodate the lower magnetic force.

The replacement heads also cause majore issues tachometers which also used the same head. When you lower the spring tension it will create a bounce in the needle. On a Tach its a catch 22 so when ever possible I will replace the flat magnet with a good used U style to achieve the proper drive.

my advice to you is not to move the tension lever to adjust your speedometer and to seek someone that has a calibration machine. If you have one then great, by all means give it a shot. All the calibration machine does is drive a cable at a fixed speed (or rpm) so if you have something you can move between the needed RPM's then you can adjust the head without issue.

Now.. with all this I'll also put another issue to rest.... These things were not designed to be tested with GPS systems... the range tolerance form AC Delco is +/- 3 - 5 mph at a given speed. Corvettes seem to be more of a 3 mph range +/-. This range will vary from head to head but in most cases the above holds true... So don't send off the speedo and get it back expecting it to be dead on at all speeds.. it could happen, but in most instances it won't! You may be good at 65 and slow at 10...

Ok.. off to bed now..

Willcox

0Willcox Corvette

Oh.. and Hi Jim, John, and Bill.. Long time.... and Jim... you are 100 percent correct... 60 mph is equal to 1000 rpms.. 500 is 30 and so on..

x0000rgw

Thanks Willcox,
You have told me what I need to know. I don't have the
equipment to re-zap the magnets, but I can tweak the spring
to show 48 MPH at 800 Rpm. Since I was showing 69 MPH
and the highway patrol man said I was going 73 then it's not off
by much. I think he just wanted to walk around the car and
kick the tires. He was a young fellow and probably never had
seen a C1 on the highway. I go up I30 about twice month so don't
know where he has been.
Thanks,
Ray

Lynn Mobley

I remember from my GM training that the instructors said speedometer cables turned 999 times per mile. Maybe this will help.