Board member that does speedo calibration?

[earl84]

I know I saw it one time, but cannot find the board member's name that does speedo calibration. I put a new drive and driven gear in my '66 C20 and now have a speedometer (need to change out the driven gear, though), and am probably going to need a calibration on my speedo. Just planning ahead.

Or, would you guys recommend a new speedometer from the aftermarket? ANy experience?

[PGSigns]

The speedometer is fixed rate. There are good calculators on line for getting the right set of gears in the trans. I was able to get the 66 right with in a couple of miles per hour using that method. I used a gps app on my phone to check it. Here is just one of them.
http://www.tciauto.com/tc/speedometer-gear-calculator/
Jimmy

[earl84]

From researching this a bit, I have found that how it actually works is that the odometer runs directly off of the drive and driven gear in the transmission that you are talking about, and the speedometer pointer changes based on a speedometer cup that rotates and sets up a magnetic field and moves the speedo based on how strong that magnetic field is. So the first thing the speedo shop has you do is get a reading on how far the odometer moves per mile traveled by comparing it to either mile markers or GPS and then changing the transmission speedo gear to get the odometer reading correctly, which is what I am doing right now.
I used the calculators and then I talked with the guys at Gear Vendors, who had the proper gears for my TH400, and ordered them. Unfortunately, the odometer is off by about 6% and the speedo is all over the place, indicating about 5 mph too slow at slower speeds, about right between 50-65 mph, and then too fast at higher than 65. So I ordered a driven gear to correct the odometer, and am just planning ahead to potentially have to send in my speedometer to have the magnetism adjusted by a speedo shop, and wanted to try to use a board member if possible.

[sambrutay]

here is some info from the internet,

Mechanical Correction Ratio Adapters, standard and metric,made to suit
from this site.
http://www.speedometersolutions.com/...eter_cable.htm

or use the calculator below to figure if you need to over drive or under drive. then find on on flee bay.



1. The speedometer head must turn 1000
revolutions to register one mile.
2. Select speedometer drive gear set from
Speedometer Gear Chart No. 847-S for
particular transmission.
3. The number of teeth in the speedometer
driven gear divided by the number of teeth
in the speedometer drive gear form a ratio.
4. To correct for various tire sizes and axle ratios,
a speedometer adapter built to a specified
ratio is required.
5. Calculate the adapter ratio required and select the
nearest ratio from Stewart-Warner chart for 666
series drive joint.
Speedometer Driven Gear
(1000) X Speedometer Drive Gear
(Tire revs per mile) x (rear axle ratio)
Example Transmission: Model RT-11609A
Tire Size: 10.00 x 20
Axle Ratio: 3.90
Speedometer Ratio: 13/6 = 2.167
Tire Revs Per Mile: 494
Speedometer Adapter Ratio = = 1.1248
Therefore use nearest adapter ratio selected from chart of 1.1250
Speedometer Adapter Ratio =
(1000) x (2.167)
494 X 3.90

[TBONE1964]

I do speedometer head calibrations and they are able to be calibrated.

It is rather simple to figure out.

If your drive a measured road mile and your odometer does not move exactly one mile then your trans (tail shaft) gears will fix that.

If it does move exactly one mile per measured mile driven and the speedometer is off, then your speedometer head needs to be calibrated.

Hope this helps,
Tom

[earl84]

Tbone, you are the guy I was looking for. I have a new driven gear on its way, and will replace that and see if the odometer is right, and then if the speedometer is correct. If not, I will be sending some business your way. Thanks.
Sambrutay, and others, Step 1 in your post is the part that the speedo shops want us to drive 1 mile and compare our odometer reading to the actual distance traveled by comparing to mile markers or GPS. That is the start point, and if it is incorrect, then you are wasting time and money buying or doing anything else, and must change the drive gear or the driven gear or both to get the odometer reading correct. Then from there, the speedo head may need calibration.

[earl84]

My new 39 tooth driven gear got the odometer within 1 mile over 100 miles, so less than 1%. And my speedo is close enough for me, it's off by about 4 mph at 30, then tapers off to read almost dead on at 65.