60th Birthday - Winter Work - '07 & '08

 

 
 

 

  

Speedometer Calibration

Here's two options for repairing your speedometer: Do It Yourself.........PDF file link at the bottom of the page OR follow the steps below and send your speedometer out for repair.

Option 1

Between the new Toyota transmission gearing, the new Nissan 4.08:1 differential and lower profile tires my speedometer was way off both with regard to speed and distance covered. Fortunately Herman addresses this by including what you need to do before sending your speedometer out for recalibration.

I had noticed in the past that there's a number on the bottom center of the Smith speedometer face. It might be 1200 or 1120 or, in my case, 1000. This number represents how many times the speedometer cable will turn one mile (5280). When you start changing things like tire size or diff gearing the cable will end up turning more or less than the required turns to be accurate.

So what you need to do is remove the speedometer from the car so you can watch the cable turn as you drive. Put piece of tape on a flat side of the cable so you can count revolutions. Now all you need to do is drive exactly one mile and count 1000 times! Ok.... that won't work so how about measuring off exactly 1/100th of a mile or 52 feet 9-1/2 inches.

Here's the steps:

  1. Remove your speedometer.
  2. Put a piece of tape on a flat side of the speedometer cable.
  3. Mark off exactly 52 feet 9-1/2 inches on the pavement.
  4. Have a friend watching your rear tire to tell you when to stop.
  5. Park your car with the rear tire centered on the start mark.
  6. Drive toward the finish mark, counting the times the cable revolves.
  7. Stop the car when your rear tire is centered over the finish mark. That's your friend's job!
  8. Record the number of turns including your best estimate of the last fraction of a turn.
  9. Do this about 4 or 5 times and use the two results that most closely match.

In my case the cable was turning about 11-3/4 times. The speedometer number on the face was 1000 which divided by 100 (we were doing 1/100th of a mile) equals 10. To be accurate my cable should have been turning 10 times in the measured distance.

So now all you have to do is go to the speedometer repair shop of your choice with your speedometer and cable and tell them how many turns the cable made in 52 feet 9-1/2 inches. If they look at you like you have two heads, pick up your speedometer and head to a shop that understands Smith gauges.

I used West Valley Auto in Reseda, Calif which specializes in British cars. Their phone number is 818-758-9500 and ask for Morris. They did a great job rebuilding the speedometer, including new gearing for the odometer and a new inner cable. And turned it around in about 10 days.

Since then I've had then calibrate my tach and this winter (2012-2013), I'm thinking about a total gauge refurbishing right down to a  re-paint of all the faces in black letters/numbers on white face. They do great work.

The proper cable is key to an accurate and long lasting gauge as the wrong cable specs can damage the internal workings of the gauge. When I spoke to Morris he asked me to measure the tip of the cable and then told me it was 1/16" too long.  He explained that basically a cable tip that's too long or thin/fat can damage the inner workings of the gauge.

So here's the specs that Morris requires:
1. The minimum tip protrusion is 1/4" and the maximum is 5/16"
2. The tip should be square and measure a minimum of .122" square to .124" square
3. He prefers that the cable bushing be nylon and not steel.

If your cable doesn't meet those specs have the cable rebuilt to the specs. It will only cost $25-$30.

And don't assume that the new cable you just bought from Moss or TRF has the proper specs..........measure it and confirm that it's correct.

Option 2

If you want to try and rebuild the speedometer yourself, you can download this PDF file.